Throughout the day, the man displayed what appeared to be an electronic transmitter with which he had said he could detonate explosives in the truck.

The law-enforcement authorities said neither they nor the man had set any deadlines, adding that he had refused to negotiate with them. Reporter Was Intermediary

Agents of the Federal Bureau of Investigation, the Federal Park Service Police and Metropolitan Police officers formed a negotiating team and dispatched Steve Komarow, a correspondent for The Associated Press who had volunteered to act as an intermediary.

After meeting several times with the man outside the truck, Mr. Komarow quoted him as saying: ''It's up to the President, it's up to the media. They have been pretending that we are not threatened every day of our lives with annihilation and whether by collusion or otherwise they refuse to give the real information about the precarious and uncontrollable situation the world finds itself in.''

On the right side of the truck was attached a sign: '' 1 Priority: Ban Nuclear Weapons.'' Visited Capital Often

Mr. Mayer, who the authorities said was active in the antinuclear movement, visited the capital often, staying at the Downtowner Motel in Northeast Washington. The hotel's owner said Mr. Mayer was ''quiet and unassuming.''

Mr. Mayer was described by the Miami Beach police as a World War II veteran who had a criminal record of several misdemeanors and drug trafficking convictions. The owner of the Miami Beach Continental Hotel, where Mr. Mayer once lived, said he was the head of an antinuclear organization called No. 1 Priority.