NEW DELHI, INDIA, April 6.-U.S. and other foreign refugees from East Pakistan said today the rebels and their antagonists, the army, had staged an orgy of mass executions in Chittagong. Burning and looting left the port of Chittagong an almost deserted wasteland, they said.

The Bengalis of East Pakistan still were fighting outside Chittagong but the city itself was in the hands of West Pakistan's army, reported some of the 37 Americans and 82 other foreigners reaching Calcutta on the British ship Clan MacNair.

Twenty-six Americans who flew into Tehran, Iran, today from Dacca told of shooting and burning, They said Americans had not been threatened and the Pakistan government was in control in the capital city.

In a broadcast over official Pakistan radio, Pakistani President Mohammed Yahya Khan said India had sent the equivalent of six army divisions into West Bengal state which borders East Pakistan.

The broadcast quoted Yahya as saying, "This concentration on our borders represents a direct threat to our security." it also said that federal Pakistani troops had again "confronted" armed infiltrators from India.

The Indian foreign ministry denied Yahya's charge. "We have taken precautions not to put any regular army troops in the area," a ministry spokesman said. He said only paramilitary border forces are stationed along the frontier.

Stories told by refugees and information gathered by reporters slipping into East Pakistan indicate Yahya's army was increasingly isolated in the major cities of secessionist East Pakistan.

After 12 days of civil war, a general picture of the strength of the opposing forces was emerging.

The West Pakistan army is in control of at least the major cities in the province, Dacca the capital, and Chittagong.

Elsewhere in the 55,126-square-mile province, and even on the outskirts of Dacca and Chittagong, the followers of Sheikh Mujibur Rahman and his outlawed Awami League were reported to be in control.

Refugees who arrived from Chittagong today talked to newsmen in Calcutta.

"The army only had to see a Bengali on the street and they shot him," said a Briton. "Chittagong is now deserted. Everyone has fled to the villages outside."

Said another Briton who was an engineer in a Chittagong jute mill: "in my own mill, people were murdered in front of my eyes. Many people were burned in their houses when the army put them to the torch. They seemed to take a delight in destruction. Machine guns were going day and night and there was shelling from gunboats offshore."

The Britons said, however, that for two weeks before West Pakistan's army arrived, Bengalis had been murdering West Pakistanis living in Chittagong. They told of corpses lying in the streets for nine days.

Leon Lumsden, an American engineer working on a U.S. aid project reported: "Soldiers just walked up to anyone wearing a lungi, the native Bengali dress, and shot them. He estimated he had seen 16 Bengalis killed,

'Charles McGinley, U.S. cultural affairs officer in Dacca who arrived in Tehran today, called life in East Pakistan "decidedly abnormal," but said, "it is accurate to say the Americans were frightened, but there was no threat to them and, as far as I know, no American deaths at all."

In Dacca, the army is now in control, and it was quiet when we last left it," he said.

Another woman who refused to give her name said, "So many of my dear Pakistani friends are now dead. They are still shooting and burning places down." She said parts o$ Dacca were "definitely in shambles. Tanks keep rumbling past homes. People are being constantly arrested."

(The State Department reported in Washington that 307 Americans have left Dacca in East Pakistan for Karachi in West Pakistan aboard five flights of Pakistan International Airways transport planes. That left approximately 450 Americans still in East Pakistan) .

Two officials of the Pakistani high commission (embassy) in New Delhi defected to India, today to express their sympathy with the Independence movement in East Pakistan.

Indian government sources said the diplomats, Second Secretary K. M. Shehabuddin, and assistant press attach Amhudul Huq, asked for and were granted political asylum in India. Both are East Bengalis.

In Karachi, the national Pakistani news agency reported that Kamal Hossain, constitutional expert of Sheikh Mujibur's banned Awami League in East Pakistan, has surrendered to the martial law authorities.

The agency said that other Awami League leaders who had gone underground were also beginning to surrender.

Hossain was believed to have wielded great influence in shaping Mujibur's speeches and in drafting his policies.

In the abortive 11-day talks in Dacca preceding the army crackdown, Hossain was one of the Awami League leaders who held daily talks with President Yahya Khan's aides.