BANGKOK, March 29.-In the name of "God and a united Pakistan" Dacca is today a crushed and frightened city.

After 24 hours of shelling by units of the Pakistan army, as many as 7,000 people are believed to be dead, large areas of the city have been leveled and burned and East Pakistan fight for independence has been put to an end.

Despite claims by President Yahya Khan, head of the country's military government, that the situation is now calm, on Sunday tens of thousands of people were still fleeing to the countryside, And Dacca's streets were almost, deserted.

But there is no doubt that the army is more or less in control of all the towns and major population centers, and that resistance among the 73 million Bengalis in Pakistan's east wing is minimal and, so far, ineffective It is impossible to accurately assess the cost in human lives, but reports beginning to filter in from Chittagong, Comilla and Jessore put the figure, including Dacca, in the region of 15,000 dead.

Military casualties are not known, but at least two soldiers have been wounded and one officer killed. The Bengali uprising seems to be over for the moment, Sheikh Mujibur Rahman, East Pakistan's popular political leader, was seen being taken away by the army, and nearly all the top members of his Awami League party have also been arrested.

Leading political activists have been arrested, others are dead, and the offices of two papers which supported Mujibur's movement have been destroyed. But the first target as the tanks rolled into Dacca on the night of Thursday, March 25, seems to have been the students.

THREE BATTALIONS

An estimated three battalions of troops were used in the attack on Dacca - one of armored, one of artillery and one of infantry. They started leaving their barracks shortly before 10 p.m.

By 11, firing had broken out and the people who had started to erect makeshift barricades overturned cars, tree stumps, furniture, concrete piping-became early casualties.

Sheikh Mujibur was warned by telephone that something was happening, but he refused to leave his house. "If I go into hiding they will burn the whole of Dacca to find me," he told an aide who escaped arrest.

The students were also warned, but those who were still around later said that most of them thought they would only be arrested.

Led by American-supplied M-24 World War II tanks, one column of troops sped to Dacca University shortly after midnight. Troops took over the British Council library and used is as a fire base from which to shell nearby dormitory areas.

Caught completely by surprise, some 200 students were killed in Iqbal Hall, headquarters of the militantly antigovernment students union, I was told. Two days later, bodies were still smoldering in burnt-out rooms, others were scattered outside, more floated in a nearby lake, an art student lay sprawled across his easel. The military removed many of the bodies, but the 30 bodies still there could never have accounted for all the blood in the corridors of Iqbal Hall.

At another hall, reportedly, soldiers buried the dead in a hastily dug mass grave which was then bull-dozed over by tanks.

People living near the university were caught In the fire too, and 200 yards of shanty houses running alongside a railway line were destroyed

MARKET RAZED

Army patrols also razed nearby market area. Two days later, when it was possible to get out and see all this, some of the market's stall-owners were still lying as though asleep, their blankets pulled up over their shoulders.

In the same district, the Dacca Medical College received direct bazooka fire and a mosque was badly damaged.

As the university came under attack, other columns of troops moved In on the Rajabagh headquarters of the East Pakistan police, on the other side of the city. Tanks opened fire first, witnesses said; then the troops moved in and leveled the men's sleeping quarters, firing incendiary into the buildings.

People living opposite did not know how many died there, but out of the 1.100 police based there not many are believed to have escaped .

MUJIBUR'S ARREST

As this was going on, other units had surrounded the Sheikh's house. When contacted shortly before 1 a.m. he said that he was expecting an attack any minute and had sent everyone except his servants and bodyguard away to safety.

Neighbor said that at 1:10 a.m. one tank, an armored car and trucks loaded with troops drove down the street firing over the house.

"Sheikh you should come down," an officer called out in English as they stopped outside. Mujibur stepped out onto his balcony and said, "Yes, I am ready, but there is no need to fire. All you need to have done is call me on the telephone and I would have come."

The officer then walked into the yard and told Mujibur: "You are arrested."

He was taken away along with three servants an aide and his bodyguard who was badly beaten up when he started to insult the officer. One man was killed-night watchmen hiding behind the fence of the house next door.

As the Sheikh was driven off-presumably to army headquarters-the soldiers moved into the house, took away all documents smashed everything in sight locked the garden gate shot down the green red and yellow Bangla Desh flag and drove away.

By 2 a.m. Friday morning fires were burning all over the city and troops had occupied the university and surrounding areas.

There was still heavy shelling In some areas but the fighting was beginning to slacken noticeably. Opposite the Intercontinental Hotel a Platoon of troops stormed the empty offices of The People newspaper burning it down along with most houses in the area and killing the night watchman.

CITY LIES SILENT

Shortly before dawn most firing had stopped and as the sun came up an eerie silence settled over the city deserted and completely dead except for noise of the crows and the occasional convoy of troops or two or three tanks rumbling by mopping up.

At noon again without warning columns of troops poured Into the old section of the city where more than I million people lived In a sprawling maze of narrow winding streets .

For the next 11 hours they devastated large areas of the old town as it is called where Sheikh Mujibur had some of Hs strongest support in Dacca. English Road, French Road, Niar Bazaar, City Bazaar were burnt to the ground .

They suddenly appeared at the end of the street said one old man living in the French Niar Bazaar area. Then they drove down it firing Into all the houses.

The lead unit was followed by soldiers carrying cans of gasoline. Those who tried to escape were shot. Those who stayed were burnt alive. About 700 men women and children died there that day between noon and 2 p.m., I was told.

The pattern was repeated in at least three other areas of up to a half square mile or more. Police stations in the old town were also attacked.

CONSTABLES KILLED

I am looking for my constables a police inspector said on Saturday morning as he wandered through the ruins of one of the bazaars. I have 240 in my district and so far I have only found 30 of them-all dead.

In the Hindu area of the old town the soldiers reportedly made the people come out of their houses and shot them in groups. This area too was eventually razed.

The troops stayed on in force in the old city until about 11 p.m. on the night of Friday, March 26, driving around with local Bengali informers. The soldiers would fire a flare and the informer would point out the houses of Awami League supporters. The house would then be destroyed either with direct fire from tanks or recoilless rifles or with a can of gasoline witnesses said.

Meanwhile troops of the East Bengal Regiment in the suburbs started moving out toward the industrial areas about 10 miles from the Sheikh's centers of support. Firing continued in these areas until early Sunday morning but the main part of the operation in the city was completed by Friday night-almost exactly 24 hours after it began .

One of the last targets was the daily Bengali-language paper Ittefaq. More than 400 people reportedly had taken shelter in its offices when the fighting started. At 4 o'clock Friday afternoon four tanks appeared in the road outside. By 4:30 the building was an inferno witnesses said. By Saturday morning only the charred remains of a lot of corpses huddled in back rooms were left.

CURFEW LIFTED

As quickly as they had appeared the troops disappeared from the streets. On Saturday morning the radio announced that the curfew would be lifted from 7 a.m. until 4 p.m.

It then repeated the martial law regulations banning all political activity announcing press censorship and ordering all government employees to report back for work All privately owned weapons were ordered to be turned in to the authorities.

Magically, the city returned to life, and panic set in. By 10 a.m. with palls of smoke still hanging over large areas of the old town and out in the distance toward the industrial areas the streets were packed with people leaving town.

By car in rickshaws but mostly on foot carrying their possessions with them the people of Dacca were fleeing. By noon the refugees numbered in the tens of thousands "Please give me lift I am old man"-"In the name of Allah help me-"Take my children with you."

Silent and unsmiling they passed and saw what the army had done. They looked the other way and kept on walking. Down near one of the markets a shot was heard. Within seconds 2,000 people were running; but it had only been someone going to join the lines already forming to turn in weapons.

Government offices remained almost empty: Most employees were leaving for their villages ignoring the call to go back to work.

Those who were not fleeing wandered aimlessly around the smoking debris lifting blackened and twisted sheets of corrugated Iron ( used in most shanty areas for roofing) to salvage from the ashes what they could. Nearly every other car was either taking people out into the countryside or flying a red cross and convoying dead and wounded to the hospitals.

In the middle of it all occasional convoys of troops would appear the soldiers peering- equally unsmiling- down the muzzles or their guns at the silent crowds.

On Friday night as they pulled back to their barracks they shouted "Narai takpir," an old Persian war cry meaning "We have won the war." On Saturday when they spoke it was to shout "Pakistan zindabad"-"Long live Pakistan."--

FAST SELLING FLAGS

Most people took the hint. Before the curfew was reimposed, the two hottest-selling Items on the market were gasoline and the national flag of Pakistan. As if to protect their property in their absence the last thing a family would do before they locked up their house would be to raise the flag.

At 4 o'clock Saturday afternoon the streets emptied again. The troops reappeared and silence fell once more over Dacca.

But firing broke out again almost immediately. "Anybody out after four will be shot," the radio had announced earlier in the day.

A small boy running across the street outside the Intercontinental Hotel two minutes after the curfew fell was stopped slapped four times in the face by an officer and taken away in a jeep.

The night watchman at the Dacca Club a bar left over from the colonial days was shot when he went to shut the gate of the club.

A group of Hindu Pakistanis living around a temple In the middle of the race course were all killed apparently because they were out in the open.

Refugees who came back into the city after finding that roads leading out of it were blocked by the army told how many had been killed as they tried to walk across country to avoid the troops.

Beyond those roadblocks was more or less no-man s land where the clearing operations were still going on. What is happening out there now is anybody's guess except the army's.

Many people took to the river to escape the crowds on the roads but they ran the risk of being left stranded waiting for a boat when curfew fell. Where one such group was sitting on Saturday after noon there were only bloodstains the next morning.

Hardly anywhere was there evidence of organized resistance. Even the West Pakistani officers scoffed at the idea of anybody putting up a fight.

"They're bugger men," said one Punjabi lieutenant, "could not kill us if they tried."

"Things are much better now," said another officer.

"Nobody can speak out or come out. If they do we will kill them-they have spoken enough-they are traitors and we are not. We are fighting in the name of God and a united Pakistan."