Under cover of pre-dawn darkness. Pakistani Army buses and trucks carried American officials past their burned-out embassy here today on the first leg of their journey home.

A Pan American jumbo jet evacuated more than 400 Americans, mostly dependents of embassy personnel and including about 60 U.S. government employes deemed to be nonessential and about 40 private citizens who wanted to leave the country.

[The plane arrived at Dulles International Airport at 8:35 p.m. Friday after making a brief stopover in Frankfurt, West Germany. Story on Page A10].

The evacuation was ordered by Secretary of State Cyrus Vance. U.S. diplomats here complained loudly about how long it took the Pakistani government to rescue them from a raging mob that set fire to the embassy Wednesday and kept them pinned down for seven hours.

Ambassador ARTHUR W. Hummel warned the departing Americans against making critical comments about Pakistani response to the continual calls for help. He said such remarks could endanger Americans held hostage at the U.S. Eembassy in Tehran as well as the embassy staff remaining here.

The Pakistani Army ran the evacuation gathering Americans in buses at 3 a.m. for the 12-mile ride to the airport in Rawalpindi. Some buses used this morning belonged to the public transporation company that had given free rides Wednesday to students and youths who attacked the embassy.

The plane carried the coffins of the two Americans killed in the rioting, Marine Cpl. Steven Crowley, 19, shot in the head while defending the embassy from its roof, and Army Warrant Officer Brian Ellis, 30, who was found yesterday burned to death in his embassy compound apartment.

For the rest of the Americans going home, the airport was a scene of tearful adults and children carrying toys for the long ride home.

More than 300 persons got on the plane here. Others were flown from Karachi, where the State Department has a regional office to handle administrative details for the Near East and South Asia.

In all, the U.S. establishment in Pakistan will be cut by more than three fourths. The International Communications Agency, which had 21 information sepcialists at four posts in Pakistan, is trimming down to five. Two American libraries, in Lahore and Rawalpindi, were destroyed by the anti-American violence that swept Pakistan Wednesday.

The Agency of International Development contingent here, already trimmed because of an aid cutoff triggered by Pakistan's nuclear explosives program, was further cut to seven. One of the four economics officers in the embassy, along with two secretaries, are also leaving.

In all, the 120-person U.S. diplomatic contingent here in the capital was cut to about 40.

The State Department ordered the evacuation shortly after about 90 persons who had been trapped in the burning embassy for more than seven hours were able to escape.

"I am not certain we would have made the evacuation plans this quickly if it was not for Iran," said a U.S. diplomat here.

Smoke still curled from the embassy today. U.S. officials for the first time allowed reporters a close look at what once was thought to be an impenetrable brick main building surrounded by a high fence.

Not one building in the 32-acre $21 million compound remained unscathed. The Pakistani government, which has played down the burning of the embassy and the killing of two American and two Pakistani employes in the mob violence, today ordered a full investigation and expressed its "strong disapproval" of the "irresponsible" behavior of the mob. But the government-censored Pakistan Times referred to the destruction as "vandalism."

President Mohammad Zia ul-Haq pledged that his martial law government would provide complete protection to all foreign diplomats here and, in the wake of attacks on a convent school in Rawalpindi and two Chritian churches elsewhere, to minority religions.

Meanwhile, U.S. Embassy officials today gave a new account of the death of the two Pakistani employes that contradicted the account given by Ambassador Hummel at a press conference yesterday.

Hummel had said the two Pakistanis died on the top floor of the embassy, "where the vault is, where the people proceeded during the last stage before they filed into the vault," the security area that protected them until the spreading fire forced them to the roof.

Today, U.S. officials said the two Pakistani employes died in a groundfloor office, apparently of smoke inhalation.

At the compound today, U.S. security officials searched for the fireproof office safes containing classified and other important embassy documents. Three hundred Pakistani troops deployed to guard the compound set up a tent city within the perimeter.