Nearly 900 Americans left here today for Western Europe to begin a massive U.S. government organized evacuation of its nationals.

The pullout coincided with the hesitant beginnings of what Iran's new revolutionary government hopes will be a return to normal. At the urging of the revolution's leader, Ayatollah Ruhollah Khomeini, Iranians started going back to work after a three-month general strike that toppled Shah Mohammad Reza Pahlavi and ended the monarchy.

Oil industry sources said most of the country's 67,000 employes in the crucial oil sector returned to work, although about 15,000 leftists, who forced the shutdown of the industry in December, refused to go back to their jobs.

As Iran slowly began to return to normal conditions, Khomeini held his first meeting with a prominent foreign official, Palestine Liberation Organization leader Yasser Arafat, Iranian radio said. It gave no details of the meeting but it had been announced previously that a PLO mission will take over the former Israeli mission here.

As the Americans began their departure, the government moved to disband the military organization whose actions led directly to the last round of violence.

Maj. Gen. Mohammed Ali Gharani announced that the Imperial Guards and its "Immortals" Division -- whose attack on Air Force cadets a week ago ended disastrously and was instrumental in sweeping away the last trappings of the monarchy -- had been dissolved.

Meanwhile, the last mass exodus of Americans -- prompted by U.S. Embassy warnings that it could no longer protect Americans in the chaotic revolutionary atmosphere -- began last night as evacuees gathered in the embassy cafeteria and the Hilton Hotel ballroom.

Kept awake by firing outside the embassy, the often frightened evacuees were escorted by armed supporters of the new government without incident to Mehrabad Airport aboard 12 buses early this morning.

But the first group took off for Frankfurt, West Germany, aboard a Boeing 747 jumbo jet more than six hours behind schedule because of a combination of confusion, red tape, mislaid passports and exhaustive baggage checks. That flight and a later one bound for Rome reached their destinations late today.

Royal Air Force aircraft -- a C130 cargo plane and a VC10 passenger jet -- flew out about 20 Britons to the Persian Gulf island state of Bahrain.

Iranian newspapers reported that 21 more pro-shah leaders were scheduled for the firing squad tonight following the execution of four Thursday night.

Two prominent generals -- Iraj Moghaddam, minister of power in the short-lived military government last fall, and Hasham Naji Nejad, a specialist in arms procurement -- were reported to have committed suicide.

An aide of Prime Minister Mehdi Bazargan said that former prime minister Amir Abbas Hoveyda and 21 other senior officials who served the shah and who had already been tried by a summary court were being retried today by a more permanent revolutionary tribunal.

An aide to Khomeini said it would be impossible for journalists to attend the trials.

Among the growing list of arrested generals was Maj. Gen. Ali Neshat, the "Immortals,'" commander, and Gen. Hassan Toufanian, vice war minister in charge of the shah's massive arms procurement program that now has ended ignominously.

At least 20, and perhaps as many as 34, generals and colonels were reported to have been purged. The bank accounts of all senior officers of all three branches of the armed forces were ordered frozen.

Symptomatic of the pressures brought to bear by the leftist-influenced rank and file was an official announcement changing the leadership of the gendarmerie and national police less than a week after Bazargan named his first choices for the jobs.

Further indications of growing friction between Khomeini's government and the extreme left was provided by a report that the ayatollah's armed marshals had arrested leaders of the Marxist Cherikhaye Fedaye Khalq guerrillas who had sought to march on Khomeini's headquarters.

Throughout the day, often scraggly, bearded, armed revolutionaries rounded up suspected members of SAVAK, the once dreaded, now disbanded secret police.

The often rough-and-ready revolutionary methods of justice have outraged many human rights advocates who were in the forefront of the struggle to overthrow the shah. The English-language Tehran Journal warned that human rights must be respected, "otherwise the new society becomes as barbarous as the old."

At the airport, an American betrayed his edginess by saying, "Let's just get ot of here before they change their minds."

Tom Aubin, 32, who worked for a local television station, captured the decline and fall of American power here that culminated in the attack on the U.S. Embassy Wednesday.

"That's not an American Embassy anymore," he said. "The Marines don't have uniforms, their guns have been taken away and armed men from another country are running around telling everybody what to do."

John Archambault, who worked for an Iranian aircraft company, said, "I was evacuated out of Vietnam four years ago, but this last week scared the living bejesus out of me. I spent 5 1/2 years in Vietnam and I don't think I heard as many bullets during the entire period as I did Wednesday night when all hell broke loose near my house."

Garry Le Mierre, 53, employed by Lockheed in a support capacity, said, "Over the last month life has been terrible. We were confined to our house all the time. I went to work, but I was never comfortable about leaving my wife at home and finally we worked out a system where she would come along."

His wife said. "Our neighbors stopped talking to us cold a week ago." She echoed a lament of many departing Americans, saying, "I think President Carter was wrong to back the shah and it kept getting worse all the time with his pronouncements on how great the shah was while he kept talking up human rights which were being violated all the time."

The evacuation airlift, which should reduce the American community to less than 2,000 from its pre-November peak of almost 45,000, was carried out today by two Pan American World Airways 747s and a U.S. Air Force C-141.

Most of the evacuees were employes of Bell Helicopter and Lockheed, which had major defense contracts with the Iranian armed forces, maintaining 1,200 helicopters and training Iranian logistics personnel, respectively.

The revolutionary guard at the airport included two youths wearing a U.S. Marine flak jacket, one of them sporting a Marine shotgun apparently "liberated" during the attack on the embassy Wednesday.

The only consolation one evacuee remarked drily, was the bite administered by an American family's dog to a revolutionary guardsman's finger.