RAWALPINDI, Pakistan, Dec. 16—President Agha Mohammad Yahya Khan acknowledged tonight that his forces in East Pakistan had been overwhelmed but pledged to continue the war against India until final victory.

In a radio speech to the nation, he also urgently called for help from the community of nations in Pakistan's struggle, but added: “We shall fight alone if we. must.”

Later, Pakistan formally acknowledged that India now controls what had been East Pakistan.

A communiqué issued here in West Pakistan said:

“Latest reports indicate that following an arrangement between the local commanders of India and Pakistan in the Eastern theater, fighting has ceased in East Pakistan, and Indian troops have entered Dacca.”

No Details Are Given

The communiqué said nothing more about East Pakistan and gave no details of the “arrangement.”

But it indicated that fighting in West Pakistan was continuing.

For the first time since the war began, military and diplomatic spokesmen failed to held, their daily evening briefing and correspondents here had no access to Pakistani officials.’

The air war in the west was apparently still in progress. Indian air raids were reported on Karachi, Sialkot and other cities with significant numbers of civilian casualties.

The President implied that he had by no means accepted the separation of East Pakistan from the West, despite Pakistan's military defeat there.

“No sacrifice will be too great to preserve this Islamic homeland of the 120 million people of Pakistan,” he said.

He also said: “To all our friends we say: stand by us and rest assured that the people of Pakistan and their armed forces will not cease their struggle until aggression is vacated and justice prevails.”

General Yahya Khan said he still intended to promulgate a new national constitution on Dec. 20 that would provide, he said, for East Pakistan as a part of Pakistan.

“But,” he said of the document, “it contains clear and unequivocal provisions guaranteeing maximum autonomy to East Pakistan, consistent with the concept of one Pakistan, for the creation and preservation of which both wings of our country have undergone untold sacrifices.”

He called for “discipline and determination” from his people and said “this is a war which has to be fought in the fields, factories and homes as much as on the battlefield.”

Hails Courage of Army

In East Pakistan, he said, the army's resistance to overwhelming odds “will go down in history as an epic of indomitable courage, reminiscent of the highest traditions of the soldiers of Islam.”

Pakistan faced a “perfidious and ruthless enemy, massively equipped and backed by a superpower,” he said, and continued:

“It is for these reasons that we have been overwhelmed in the eastern sector, but a temporary setback in one theater of war does not by any means signify the end of the struggle.

“We may lose a battle, but final victory in this war of survival shall, Inshallah [God willing], be ours.”

He said Pakistan was “fighting a naked imperialist challenge which threatens not merely Pakistan but the whole of this region.”

He thanked China and the United States for their support, saying: “It is indeed heartening that these two great powers have raised their voices so firmly in the cause or peace and justice.”

The Government's evening communiqués said nothing about Indian reports of major battles in West Pakistan, one in northern Kashmir and the other in Sind Province of southern Pakistan.

The air communiqué said an Indian railroad station had been attacked.

Last night, Indian bombers attacked Karachi, Pakistan's largest city, and reportedly caused more than 50 civilian deaths. One of the planes was said to have been shot down by ground fire.