Wherever the sullen knots of Libyan diplomats gathered yesterday to talk about their expulsion by the State Department, they were never out of sight of the cheerful countenance of their leader, Col. Muammar Qaddafi, who beamed down reassuringly from pictures and posters on nearly every wall in the embassy.

The smiles of revolutionary confidence seemed somehow inappropriate. The representatives of Libya, which supplies 10 percent of America's import oil needs, had been given a week to get out of the country.

But unlike a year ago, when Iranian diplomats had hastily evacuated their embassy, quickly thrown their clothes into suitcases and summarily closed out their personal affairs in order to comply with a 24-hour expulsion order, calm prevailed at the Libyan embassy (formally called the People's Bureau) at 1118 22nd st. N.W. The 30-odd Libyan diplomats had time to reflect on what went wrong and whether there was anything they could have done to avoid the expulsion.

"We still do not have instructions from people back home as to what we must do," said El Mehdi El-Khabat, Libyan people's committee member for consular and commercial affairs at the bureau. "I am not very happy. We from our side have done everything possible to create a very good atmosphere [with the U.S. government].

"But, unfortunately, it did not work."

He glanced from beneath sleepy-looking eyes at the particularly inspiring portrait of Qaddafi -- the leader was at his relaxed and confident best -- which covered the better part of one of his office's walls. The smile was infectious and El-Khabat brightened.

"But my wife is very, very happy. But she does not understand politics," he told a reporter. "You understand."

The State Department accused the Libyans of unspecified "unacceptable conduct" at the Peoples Bureau and cited general Reagan administration concern with Libyan support for "international terrorism."

According to El-Khabat, many of the 70 non-Libyan bureau employes -- they include Filipinos and Arabs from other countries as well as several Americans -- cried yesterday when they learned about the expulsion.

"Many asked if they could come with us to Libya," El-Khabat said. "About five or six," he answered when asked how many. One of them, he said, was an American.

But he said they would probably remain behind.

Some may continue to work for the Libyan government if the State Department grants Libya permission to open an office in a friendly embassy. Iran was allowed to establish an interest section in the Algerian embassy.

"We have almost 5,000 Libyan students here," he said. "We have students in the West and Midwest -- California, Colorado, Michigan, Indiana and Ohio."

All of them receive government stipends to cover tuition, books, medical care and living expenses. Those funds are normally disbursed through the People's Bureau.

"We hope there is nothing to be affected by our absence," he said.

The Libyan government also operates a tuition free school in its old embassy on Massachusetts Avenue for the children of the several hundred Libyan students in the Washington area.

El-Khabat said that the bureau must now abandon its plan to admit any Moslem in the area seeking an Islamic grammar school education.

"We may not be able to continue to operate the school after we leave because it is on tax-free property. If the People's Bureau is not here, the State Department says we may not be able to operate it in the District of Columbia," he said.