HRIDIYPUR, INDIA. - I walked across the border from West Bengal in India to East Pakistan just beyond this tiny frontier village with a group of Bangla Desh freedom fighters on June 14.

It was a long hot walk in the sun from the Roman Catholic mission on the Hridiypur side, but fortunately I had an Indian umbrella bearer who walked at my side and kept most of direct rays off my head.

It also was by no means the first time a Western newsman has been taken on a similar stroll in the sun. The mission fathers and sisters seemed all too familiar with press excursions across the frontier.

Freedom fighters send out scouts every day to reconnoiter the other side. If all is quiet, considerable traffic back and forth ensues.

On Monday, however, scouts reported a Pakistan Army unit of 100 men was operating in the East Pakistan village of Bhobarpura just across the border.

They were said to be armed with rifles, machine guns, and mortars, and going from house to house searching for money, gold, or valuables.

We could see the nearest buildings of the village just across a wide field. Since the freedom fighters totaled 40 men, and were only lightly armed, we decided it was not prudent to proceed farther.

I walked forward to the most advanced freedom-fighter outpost. It was perhaps a half mile beyond a stone roadside marker that said "Pakistan."

The outpost was manned by two soldiers in brown uniforms and helmets. One had a rifle, the other a light machine gun. They were quickly joined by two other fighters from across the road.

BIRDS WERE SINGING

All was quiet. No firing. No voices in the distance. No sounds of looting up ahead. You could hear birds in the rice fields and mango groves.

Someone suggested pictures. The soldiers, who by this time had grown to nine, quickly lined up with weapons at ready. Posing for photographs seemed routine to them. Then I stood with them and had my picture taken. It was all most unwarlike.

Yet at the same time, the atmosphere was perfectly serious, too. Shots are often exchanged between freedom fighters and Pakistan Army soldiers. Men on both sides do get killed at just such places.

The leader of the freedom unit was 24 years old. He spoke English. He wore a wraparound sarong and sandals. Some of his soldiers looked twice as old.

ONCE A REGULAR

The leader said he was once in the regular Pakistan Army in West Pakistan. He was sent to East Pakistan and crossed into Indian territory two months ago.

In addition to vintage rides, one soldier had a mortar tube and several mortar rounds. Some of the men were former Pakistani soldiers, but others were just farmers in bits of uniforms donated by Indian friends .

"He knows only how to fire a rifle," said the leader, pointing to one of the group.

Asked how long it has been since they had operated inside Pakistan territory, they all started talking at once.

"I have been across as far as Kushtia, 60 miles from here," said one.

"I have been in three times in the past month," said another. "We fight and we attack them. A few days ago we killed 20 Punjabis in this area. We had no losses,"

NIGHT FIGHTS PREFERRED

None of the Bangla Desh fighters made a move to attack an enemy unit reported just a few hundred yards farther up the road.

"We are only 40," they said. "We prefer to fight at night."

Freedom fighters are not very effective, at present, due to lack of arms, ammunition, and training. They are trying to rectify all such shortcomings. Serious training in special camps is under way.

Meanwhile, even under present limitations they help remind people in East Pakistan that the Bangla Desh cause still lives and some resistance continues.

They go back and forth in bands of 5, 10, or 15. Often they wear no uniforms. They disguise themselves as farmers or refugees.

REFUGEE FARMERS

Many refugee farmers on the Indian side of the border wait for an all-clear signal from freedom-fighter scouts before going back to Pakistan to work their nearby fields. A group of about 50 such farmers sat under a tree and watched me take my international walk. On the return trip, I stopped for a chat .

"Most days we can go across to till our fields," they said. "But today Punjabi soldiers are there and we must wait."

Although this particular jaunt had its comic aspects, freedom fighters are a serious symbol of East Pakistani determination to resist what they regard as deliberate government mistreatment.

SPECIAL MANGO GROVE

A short distance beyond where we stood is a mango grove where the free Bangla Desh government was officially established on Pakistan soil on April 17. It is proudly referred to as "Mujib Nagar," or Mujib's capital.

"Mujib" is a popular name for Sheikh Mujibur Rahman, leader of the Awami League which won all seats in East Pakistan elections late last year.

Moreover, a group of freedom fighters returned from a raid last week with three "enemy" bodies, including one major and a captured jeep.

District freedom fighters are reported to comprise between 100 and 200 men, some of whom have gone back to East Pakistan permanently to fight and sabotage.

FRONTIER OPEN

Going back and forth is still an easy process since Indian officials consider the border open, and Nadia district has only a dozen police posts along its entire 135-mile frontier .

On the Pakistan side, however, the Army has built trenches and bunkers at some points. It reportedly is setting up a cleared strip five miles deep to control freedom-fighter incursions and perhaps refugee outflow as well.

One of the chief entry points at Shikarpur, 60 miles north, has been closed, and refugees must detour elsewhere.

Official sources here say five full divisions of the Pakistan Army - about 125,000 men - have been moved from West Pakistan to the east. This is at least partly to offset the freedom-fighter threat in addition to restoring order and intimidating or even ejecting unwanted segments of the East Pakistan population.

EASTERNERS WEEDED OUT

The belief here is that no more soldiers or officers from East Pakistan remain in the Pakistan Army today. They either have deserted to support the Bangla Desh independence cause - or have been weeded out by West Pakistan officers as security risks.

Of more than 5 million refugees in India, the great proportion are farmers and small shopkeepers, little inclined to participate in the fighting. But there also are sprinklings of teachers, students, members of Parliament, and high-ranking government officers who are helping keep Bangla Desh resistance alive.