A pirate radio broadcast by Poland's suspended Solidarity trade union was interrupted tonight just as the union's most wanted underground leader was about to speak.

Shortly thereafter, several downtown streets were cordoned off and police diverted traffic.

Radio Solidarity had gone on the air promptly as advertised at 9 p.m. local time--carefully timed for the eve of May Day celebrations organized by the ruling Communist Party. But the signal went dead when the speaker introduced Zbigniew Bujak, chairman of the union's powerful Warsaw branch who escaped detention after the declaration of martial law last December.

When the underground radio station was inaugurated two weeks ago, it was regarded as a propaganda triumph for Solidarity and an embarassment for the martial-law authorities. Over the past few days, special detector vans have been seen in various parts of Warsaw in apparent efforts to prevent a repeat.

The second broadcast began with the same signature tune as the first--a popular song sung during Nazi occupation in World War II.

A member of Solidarity's Warsaw leadership, Zbigniew Romaszewski, who said he was responsible for the station, called for a boycott of May Day celebrations: "The red flag which they carry in their parade will bear the workers' blood from Poznan the site of riots in 1956 , the Baltic coast where protesting shipyard workers were killed in December 1970 , and those who died in the war declared on their own nation" last December.

He then introduced Bujak and the taped transmission faded.