KARACHI, Pakistan, March 1 —President Agha Mohammad Yahya Khan announced today the indefinite postponement of the opening of the National Assembly two days before it was to begin drafting a constitution to return Pakistan to civilian rule.

General Yahya said in a statement broadcast by the Pakistani radio that he was acting “with a heavy heart” be cause of a “political confrontation” between leaders of East and West Pakistan.

“It has cast a shadow of gloom on the entire nation,” he said of the dispute over East Pakistani demands for full regional autonomy.

The postponement of the session, scheduled to begin Wednesday in Dacca, East Pakistan, was demanded yes by. Zulfikar Ali Bhutto, a former Foreign Minister, who heads the principal West Pakistani party in the Assembly. His leftist Pakistan People's party won 81 of the 313 seats in the election for the Assembly last December.

Mr. Bhutto, who said last month that his party would boycott the Assembly unless the party's point of view was assured consideration, announced yesterday that if the Assembly met without members of the People's party he would call a general strike in West Pakistan.

Mr. Bhutto's East Pakistani rival, Sheik Mujibur Rahman, would be expected to have the upper hand in the Assembly, for his Awami League holds 167 of the 313 seats—a comfortable majority.

Ever since the December elections, the Sheik has been contending that the constitution should be based on the Awami League program, since the central Government and army are dominated by West Pakistanis. The country has been ruled under martial law since General Yahya assumed the presidency in March, 1969, after the overthrow of President Mohammad Ayub Khan.

The Awami League wants the federal Government to look after only foreign affairs, defense and some economic questions, with safeguards to prevent the flight of capital from East to West Pakistan. It wants foreign trade, foreign aid and foreign exchange to come under the five provinces— the four provinces of West Pakistan and the one of East Pakistan. The two parts of the country are separated by more than 1,000 miles of Indian territory.

The yielding of these powers to the provinces has been denounced as unacceptable by the People's party, and Mr. Bhutto has said he needs more time to come to an understanding with the Sheik. The President said that he would call the Assembly into session after a “reasonable understanding” among political leaders on the charter.