A Washington lawyer who represents the Libyan government handed over subpoenaed office records relating to his client yesterday to an Alexandria federal grand jury investigating an alleged assassination plot in this country by supporters of Libyan leader Col. Muammar Qaddafi.

Richard C. Shadyac and his lawyer son, Richard C. Shadyac Jr., made separate brief appearances before the grand jury, and later declined to say what they were questioned about. Shadyac said they were not asked about confidential exchanges between them and their client.

"I think it's harassment," Shadyac said just before he was called in to the grand jury. "I've represented them the Libyan government since 1976, and except for . . . one case in Philadelphia, I've never seen any substance" to allegations made by U.S. investigators, he said.

Two Libyans were arrested in Philadelphia this year after ordering two bulletproof vests and four handguns with silencers from an FBI undercover agent. After pleading guilty to unlawful weapons possession, they were sentenced to prison.

The Alexandria grand jury is investigating allegations that supporters of Qaddafi plotted to assassinate at least three Libyan dissidents in this country, according to sources familiar with the probe. FBI agents delivered subpoenas to 16 Libyans in Virginia, Colorado, Michigan and North Carolina the last week in May.

All who were subpoenaed refused to testify before the grand jury, invoking their constitutional right against self-incrimination. Three were then granted immunity and ordered to testify by U.S. District Court Judge Albert V. Bryan Jr.

Two days after their appearance, all 16, who were then represented by Shadyac, were notified by the Justice Department that they were targets of the federal investigation, Shadyac said yesterday.

Government prosecutors then filed a motion saying Shadyac should be disqualified from representing the pro-Qaddafi Libyans because he had a conflict of interest. Prosecutors refused to say why Shadyac might have a conflict.

The 55-year-old lawyer said yesterday he withdrew to avoid "even the appearance of a conflict." He continues to represent the Libyan government, he said.

Shadyac received a subpoena last Friday to appear before the federal panel and bring all his records relating to his client, including bills and lists of meetings for the last five years. Shadyac Jr. said he received his subpoena Tuesday.

Shadyac said all the Libyans subpoenaed last month were either students or employes of the Libyan Arab Jamahriya Inc., a Libyan student center in McLean. The center was established after the Reagan Administration expelled Libyan diplomats in 1981.