Washington's top FBI agent was late for party Saturday night after he discovered that a stranger was in his official government vehicle in the FBI's parking lot here.

Nick F. Stames, the special agent in charge of the Washington FBI field office, said he was going to his car to pick up a tie to wear to the party honoring law enforcement officials who worked on the "Sting" undercover operations here. Stames said when he saw someone in his car, an unmarked 1977 Chevrolet Impala, he drew his gun, walked up to the car and identified himself.

He said later that to convince the suspect it was an FBI car. He told the man to open the glove compartment where police radios and other equipment were hidden.

Assisting Stames in the arrest outside the old Post Office Building here was the number two man in the hierarchy of the FBI office here. Joseph Corless, the assistant special agent in charge.

The FBI identified the suspect, who was described as "rather embarassed" by the whole ordeal, as Wilmar Berrian Jr. The man, whose age and address were not immediately available, was charged with a misdemeanor offense of tampering with an automobile.