Declaring that "silence has been an ally of apartheid," Sen. Lowell Weicker Jr. (R-Conn.) yesterday apparently became the first senator arrested in an act of civil disobedience as he joined demonstrators outside the South African Embassy to protest that country's policies of racial segregation.

"Apartheid exists not because of a few South African political leaders," said Weicker, 53, who was arrested with five others. "It exists because a whole world, and that includes us, tolerates it by silence."

The liberal Republican senator, who was charged with demonstrating within 500 feet of an embassy, a misdemeanor, said there was no difference between the silence that "envelops" the plight of black South Africans and the silence "which wasted yesterday's European Jew."

Weicker's arrest, his first, he said, adds his name to the growing list of more than 650 public officials, clergy and others, including 16 House Democrats, arrested here and elsewhere since protests against apartheid began at the embassy Nov. 21.

U.S. senators have been indicted and gone to prison for other offenses, according to associate Senate historian Donald A. Ritchie, but Weicker is apparently the first senator ever arrested in an act of civil disobedience, he said.

The senator was handcuffed during his arrest and later fingerprinted at the D.C. police department's Second District, where he was later released after accepting a misdemeanor citation.

Weicker said he would be willing to spend a night or longer in jail and to go to trial for his embassy protest actions. So far, however, charges against most of the demonstrators, including all those arrested here, have been dropped.

"The South African and U.S. governments don't want the issue before the American people," Weicker said.

The ongoing demonstrations, organized by the Free South Africa Movement, seek to secure the release of South African political prisoners, start discussions on a new constitution that would allow blacks to share power and end the Reagan administration's policy of "constructive engagement" with the Pretoria government.

Promising support for legislation that would impose economic sanctions on the South African government, Weicker called on President Reagan to take "even some small steps" to push for change in the white minority-ruled nation. He said constructive engagement -- maintaining good relations with South Africa while seeking gradual reforms -- "has had its chance and failed."

The price for that failure and the U.S. "romance with evil," he added, "has been in the coinage of dead, dying and imprisoned blacks in South Africa."

While marching earlier with about 55 other protestors, Weicker said he had lived through the turbulent civil rights and antiwar demonstrations of the '60s without getting arrested. He was seeking arrest now, he said, because, "I've spent 15 years in the Senate listening to how we have to try to work this thing out quietly."

Weicker, who was arrested along with four Philadelphia-based members of the Women's Strike for Peace group and District community activist James Early, took note of the upcoming presidential inauguration. But he called U.S. support for South Africa "a total contradiction" with the inaugural celebration and what it represents.

"I'll feel a lot better sitting up there Monday having done this today," Weicker said.