Authorities tonight announced the capture of Solidarity underground chief Zbigniew Bujak, ending his sometimes sensational 4 1/2-year career as Poland's most wanted fugitive and dealing a major blow to the political opposition.

A short dispatch by the official news agency PAP reported Bujak's arrest without revealing where or when he had been detained. The report said he had been "carrying out activities aimed to overthrow the constitutional system" and was wanted under a warrant of the military prosecutor's office.

The arrest of the popular, 31-year-old former factory worker was a major setback both for Solidarity's surviving undergound organization and the broader opposition movement, which had looked to Bujak as a symbol of defiance to communist authorities. At the same time, the action appeared to hand the government of Gen. Wojciech Jaruzelski a major political and propaganda victory only four weeks before a communist party congress.

Jaruzelski and his government's security forces had been repeatedly embarrassed by their failure to capture Bujak, the former head of Solidarity's Warsaw chapter who went underground when Jaruzelski declared martial law to suppress the union on Dec. 13, 1981.

Bujak, the highest-ranking Solidarity leader to escape the military and police dragnet in the days after the crackdown, soon took over the chairmanship of the underground Temporary Coordinating Commission (TKK). Since then, he has been a chief organizer of strikes, demonstrations, boycotts of official elections and other activities that have disrupted Jaruzelski's campaign to "normalize" the country.

Already famous as a Solidarity leader during the union's 16 months of legal activity, Bujak bedeviled the government with a stream of statements, pirate broadcasts and interviews from the underground. By the time of his arrest, he had become a kind of folk hero for Solidarity loyalists, arguably outranked in prestige only by Lech Walesa, the union's former chairman and still active spokesman.

In a statement tonight, Walesa described Bujak as "one of the most outspoken and bravest fighters for citizens' rights and for the cause of Solidarity."

Bujak's detention leaves only a handful of TKK members fugitive, most of them almost unknown to the public. A much larger group of opposition activists now live and work openly while engaging in occasional clandestine activities. More than 250, including two other prominent TKK members, are now imprisoned.

The TKK leadership, once an important threat to communist authorities, has been reduced in the last year to a largely marginal political role and appeared to rely heavily on Bujak for legitimacy.

The ruggedly handsome, articulate worker rocketed to prominence at the age of 26 after he was elected leader of pro-Solidarity workers in the huge Ursus tractor factory outside Warsaw in August 1980. He subsequently took over chairmanship of the Warsaw branch of the union and emerged as a close ally of Walesa.

In an interview with Newsweek magazine last year, Bujak acknowledged the shrinking role of the underground leadership, particularly after a call for a general strike last July brought only a small response. He said, however, that 50,000 to 70,000 Poles still participate in clandestine activity and that 200,000 are occasional supporters.

The TKK mounted a major effort last fall to persuade Poles to boycott parliamentary elections and carried out an independent survey of the turnout. Bujak subsequently told western reporters that 6,000 persons had worked on the survey and claimed that 40 percent of Poles had boycotted the elections, far more than the 22 percent reported by the government.

Though the TKK appeared less active after the elections, Bujak told western interviewers that he was determined to remain a fugitive as long as possible and was prepared to accept the consequences of arrest. "I am ready for a long struggle, a fight that may end up in anything, even jail," he told Newsweek. "I expect it to last 10 to 15 years. Such insane gestures must be performed in order to win something."