The night sky outside British Airways Flight 190 from Dulles Airport to London was turning pale with dawn. Iranian diplomat Mohammad Lavassani watched silently for a moment; then, 37,000 feet above the slate-gray Atlantic, he began his morning prayers to Allah.

All around him, Iranian men, women and children slept, exhausted by the 36 hours they had spent wrapping up their lives in Washington after President Carter severed diplomatic relations with Iran.

Softly, Lavassani chanted his prayers, bowing awkwardly three times in his cramped seat, his thin face gaunt with fatigue. Then he sat back and watched the sun edge brilliantly into the morning.

"We were ready to be taken hostage ourselves," said Lavessani, a political and economic consultant to the embassy. "We hoped the American government would take us hostage. We had already decided that we would begin fasting right away if that happened.

Even though they had been denied that chance, Lavassani and about 40 other Iranians aboard the plane still were returning home in triumph.

After months of serving as mere figureheads in Washington while all the important diplomatic action was going on in Iran, after months of feeling they had contributed nothing substantial or symbolic to the Islamic revolution, they were eagerly anticipating a heroes' welcome when they returned to Tehran late today.

All had been forced to make sacrifices in leaving. Several had tried frantically -- and unsuccessfully -- to sell their cars after they heard Monday afternoon that they would have to leave.

Others had been forced to give their furniture to friends because it could not be shipped home, or to sign power of attorneys so that their effects could be sold. They had gathered up their children, at least nine of them, including one who was 12 days old, and left.

Ali Agah, head of the embassy as charge d'affaires, had even been forced to leave behind his American wife and twin children, aged 19 months. Agah, who had lived in the United States for 14 years, mostly in Washington, said he hoped she eventually would join him in Tehran. But when asked when they would be, he could only answer sadly, "You will have to ask her."

Still another diplomat, who had served in the Iranian consulate in Chicgo, sought permission to remain because his wife was seriously ill with cancer. She already had undergone $30,000 worth of treatments, all of it paid for by the Iranian government, Agah said. But permission had been denied and now both were on board, bound for Tehran.

"During World War II, you gave the ambassador of Japan 24 hours to leave and the rest of the staff two weeks," Agah said. "I thought that is what they would do to me, make me leave and let the others stay to finish our affairs. But everyone had 36 hours. That's all."

Now, Agah said, all the embassy's affairs were being left in the hands of the Algerian Embassy -- including the responsibility for looking after the 300,000 Iranians Agah says are in the United States, including 56,000 students.

As the massive 747 lumbered through the night and into the morning, children sprawled across the seats in a section reserved for the Iranians amidships. Of the 15 women, only three wore western-style dress: skirts with heels, their heads bare. All the others were more conservatively dressed and had covered their heads with rousaries, the traditional Iranian-Muslim scarves.

The men, who talked softly or stared silently ahead for the first few hours before dozing off, wore sports coats or shirts open at the neck. For a while, one little girl played with a large plastic doll with bleached blond hair. Then the child fell asleep, too, the doll clutched in her arms.

"I feel sorry for them," said Myles Farrington, a Florida financial consultant on board for a business trip to London. "They're just people, after all. In the final analysis, they're just people. It has to be hard, no matter what they say."

Indeed, as the first few Iranians arrived at Dulles for the 9 p.m. flight Tuesday, what had seemed to be an impersonal diplomatic war took on a deeply personal note.

Hecklers cheered their departure, and the exhausted diplomats and their families seemed cowed, even shamed, rushing head down through the crowds that included a few well-wishers. When one blond American woman leaned forward to jeer a diplomat who stood in the glare of television camera lights, a small Iranian child in the arms of her mother reached out a tiny hand and, transfixed, fingered the woman's gleaming hair. She turned, smiled at the infant, then turned back to continue jeering. h

On the plane a 5-year-old Iranian boy, with rich dark eyes and Adidas tennis shoes, wandered down the aisles chatting with nearby American passengers.

"I'm going to where I was born," said the boy, bound for Iran, in flawless English.

"Were you born in Iran?" asked an elderly woman who was listening.

"No, I was born in London," he said.

In fact, for many of the Iranian diplomats, the forced departure from Washington to Iran was less a homecoming than a return to a homeland they had forsaken for other countries, including the United States, many years before.

Some had long been active in antishahist efforts and had left Iran to avoid harassment or, they said, imprisonment.

Most, however, had come to study in the United States years ago. Many of them had married and reared children who considered themselves more American than Iranian, and now were facing something more akin to exile than deportation.

Yet, all said they were eager to return.

"Believe me, all of us are happy we are leaving," said Ahmad Khorasanizadeh, a 24-year-old who earned his bachelor's and master's degrees in industrial engineering at the University of Oklahoma between 1973 and 1979. "We had forgotten what it meant to be Muslim, to give of yourself for your neighbor. The revolution taught us that Islamic truth again. Many who had stopped practicing Islam have returned to the faith. It had been forgotten. Now we remember."

When the plane finally landed at London's Heathrow Airport seven hours after takeoff and the 310 passengers began to exit, an Iranian woman, her head covered with a rousary, bent to pin a fresh disposable diaper on a small child.

"That's a beautiful baby," said an American woman who stood patiently waiting for the Iranian to finish and clear the aisle. "She's been so good." w

"Yes," said the Iranian woman. "I think she knows she is going home."