The FBI conducted extensive surveillance of the Libyan student center in McLean as part of its investigation into an alleged plot by some supporters of Libyan leader Col. Muammar Qaddafi to assassinate at least three Qaddafi dissidents, according to sources familiar with the investigation.

A federal grand jury in Alexandria has heard testimony from five pro-Qaddafi Libyans, and a source said the jury was examining the activities of Libyans connected to the student center. Investigators also are looking at the activities of a Libyan student in the Washington area who recently came to the United States but who is not an official at the center, sources said.

The center, scene of a brief 1982 takeover by Libyan students opposed to Quaddafi, is run by a council of four Libyan students. The chairman of the council, Salem Zubeidy, was among the 15 to 18 Libyans from Virginia, North Carolina, Michigan and Colorado subpoenaed by the grand jury after FBI agents conducted surprise interviews around the country Tuesday night.

Richard C. Shadyac, a longtime attorney for Libyan interests in the country, said yesterday that the center is "as clean as a whistle" and that there is no assassination plot. "Every perception we have is that the grand jury is trying to put something together," he said.

The grand jury finished hearing two days of testimony last night and all of those subpoenaed were excused. No indictments were returned.

Earlier yesterday, two Libyans subpoenaed before the grand jury were given immunity from prosecution, in addition to two Libyan students from the Washington area and a student center employe who were granted immunity Wednesday.

The five were granted immunity after they refused to testify before the grand jury and invoked their constitutional right against self-incrimination. They then went back before the grand jury for additional questioning.

Sources familiar with the probe said law enforcement officials decided to disclose the months-long secret investigation in part to disrupt the alleged plot and to use the grand jury to gather additional evidence.

Center leader Zubeidy, who was not granted immunity, declined to comment yesterday. In an interview about a year ago, he said he believed the U.S. government has secretly placed electronic listening devices in the center. "I know occasionally the mail of certain students is tampered with and their phones tapped also," Zubeidy said.

The student center was opened in a three-story office building in McLean less than a month after the Reagan administration closed the Libyan Embassy in Washington in 1981 and expelled Libyan diplomats, accusing them of supporting international terrorism. The Libyans also maintain a mission at the United Nations in New York City.

Spokesmen for the center said it was established to provide financial and other assistance to Libyan students in the United States. Libyan dissidents have alleged that the center is a front used by the Libyan government to promote terrorism in the United States and to harass and control Libyans living here, particularly students -- allegations denied by center officials.

Sources said yesterday that the FBI warned the three Libyan dissidents believed to be assassination targets of the alleged murder plot, and two of them are now outside of the United States. A source said their departures were not directly related to the alleged plot. The identities of the three dissidents have not been disclosed.