Pedro Joaquin Chamorro Jr., the editor of Nicaragua's sole opposition newspaper, La Prensa, has gone into voluntary exile in Costa Rica, charging that censorship and travel restrictions by the leftist Sandinista government have made his life in Managua "impossible."

Chamorro, 33, said in an interview yesterday that the Sandinistas' increasing repression of their domestic critics, combined with anonymous death threats, have caused many opposition political figures to consider leaving. "I know about 20 that would leave if they could," he said.

He added that Nicaragua's economic situation is so bad that he can make more money in 45 minutes of work on an article for publication abroad than he can for a week's labor in Managua.

Chamorro, who also is a leader of the opposition Social Democratic Party, charged that the Nicaraguan government is trying to retain dissidents in the country to prevent them from speaking out abroad against government abuses.

He said he had been told he was "first on the list" of dissidents to be killed in the event of any U.S. military action against Nicaragua.

Censorship of La Prensa has become "very, very bad," Chamorro said.

"We're not running the paper anymore. What we do is fill it up," often with wire service stories of little local interest, after Sandinista censors remove most of the stories about Nicaraguan events, he said.

In response, Nicaraguan Ambassador Carlos Tunnermann said few people have traveled in and out of Nicaragua so often.

"It is not exile, because he has not been expelled . . . . He admits to an economic interest in leaving," Tunnermann said.

The ambassador denied any systematic harassment of dissidents and said U.S. military action "is a threat to all of us, but we will remain and defend the country."

Chamorro said that he left Nicaragua for a public appearance in Japan on Nov. 15 after four days of bureaucratic delay over his visa, which suddenly had been canceled, and that he succeeded in leaving only because Japanese diplomats in Nicaragua intervened on his behalf directly with Interior Minister Tomas Borge.

He said he then traveled to Spain for another appearance and was now on the way to Costa Rica. He said he thought that if he returned to Nicaragua he would not be allowed to leave again.

Chamorro said his wife and four small children have lived in San Jose, Costa Rica, since January "because I didn't want to give them a Marxist-Leninist education."

He said current travel restrictions made it impossible to visit his family as often as he wanted, while censorship and threats pointed to a deteriorating political situation.

He said he thinks of his departure as "an overnight stay" and said he is willing to return to Nicaragua "tomorrow, as soon as these restrictions are lifted."

Chamorro's departure widens the chasm in the Chamorro family, one of Nicaragua's most powerful and certainly the most publicly torn by the decade of conflict.

Chamorro was a writer at La Prensa when his father, the publisher and an outspoken critic of the late dictator Anastasio Somoza, was murdered in 1978, presumably by Somoza agents.

After the Sandinistas took power in 1979, Chamorro's mother, Violeta, was a member of the governing junta. But she resigned in 1980 and Chamorro, the editor of La Prensa, gradually became a critic of the Sandinistas' leftward movement. Eventually he split with others in the family and converted La Prensa into the major and finally the sole voice of opposition.

His brother Carlos became head of Barricada, the government's official organ, and his uncle Xavier runs El Nuevo Diario, which also backs the Sandinistas.

La Prensa continues to be published.