An additional 98 antiapartheid demonstrators, including the Rev. William Sloane Coffin, were arrested outside the South African Embassy yesterday, the largest group yet to seek arrest since the ongoing protests began there Nov. 21.

Yesterday's "messengers" to the embassy included Baptists, Methodists, Presbyterians and an Armenian bishop who said they joined the demonstration to show the U.S. and Pretoria governments that the religious community is "deeply disturbed" about policies in South Africa and U.S. "constructive engagement" with those policies.

"Apartheid is to Christianity what the Ku Klux Klan is to Christianity," said Coffin, minister of Riverside Church in New York City, which sent down a busload of protesters.

"It is a ridiculous and obscene act not to give the vote to someone like Bishop Desmond Tutu," Coffin said, referring to apartheid policies that deny black South Africans, including Nobel Peace Prize winner Tutu, the right to vote.

Ellen Kirby, representing Global Ministeries of the United Methodist Church, said her church had long ago pronounced apartheid "legalized racism in its cruelest form." She said she hoped participation by churches would become a major force in the growing antiapartheid movement.

It took six police vehicles, including wagons, buses and a van, to transport the demonstrators to the Old Superior Court Building near police headquarters for processing. All were charged with demonstrating within 500 feet of an embassy, a misdemeanor.

Yesterday's arrests brought the total number of arrests at the embassy to 627. Since a Nov. 21 sit-in at the embassy, more than 1,100 protesters have been arrested in 17 cities during demonstrations against apartheid.

"We said 10 weeks ago that we would be out here, and 10 weeks later, we're still here," said Randall Robinson, executive director of TransAfrica, the Caribbean and African lobbying group that is spearheading a Free South Africa Movement and organizing the demonstrations.

"We will be here winter to spring, spring to summer," Robinson said.

About 175 protesters participated in yesterday's demonstration, chanting and waving signs as they marched in a block-long procession a block south of the embassy on Massachusetts Avenue NW.

A persistent theme in the peaceful protests has been to pressure the white minority-ruled South African government to extend full political participation to that nation's black majority and to persuade the Reagan administration to take a more aggressive role in speeding political, economic and social change for black South Africans.