A former U.S. senator was arrested here yesterday and the dean of Columbia University's Graduate School of Journalism was arrested in New York during increasingly vigorous protests against apartheid in South Africa.

In Washington, organizers of the Free South Africa Movement expanded the antiapartheid protest at the South African Embassy from three arrests a day to 12 and said that the number of arrests will grow "when the New Year comes."

Former South Dakota senator James Abourezk, founder of the American Arab Anti-Discrimination Committee; John Sturdivant, vice president of the American Federation of Government Employees, and the Rev. Sean McManus, board member of the National Irish Caucus, were among 12 persons arrested late yesterday afternoon for demonstrating within 500 feet of the embassy, bringing the total number of arrests there to 48.

In New York, demonstrations in front of the South African consulate resulted in 16 arrests, including Osborn Elliott, dean of Columbia University's Graduate School of Journalism and the former editor of Newsweek; Leo Corbie, chancellor of student affairs at the City University of New York; Andy Cooper, publisher of The City Sun, a news weekly, and three state legislators.

So far, 71 persons have been arrested during New York's antiapartheid demonstrations, which began 13 days after a Nov. 21 sit-in at the embassy here launched the Free South Africa Movement. Yesterday's arrest total was the largest in a single day.

Actor and writer Ossie Davis, Rep. Mary Rose Oakar (D-Ohio) and Puerto Rico Gov. Carlos Romero-Barcelo joined more than 200 demonstrators on the picket line a block south of the embassy here yesterday. None was arrested, but all pledged their support for ending racial segregation in South Africa.

"American overseas investments do make a difference," said Davis, a founder of Artists and Athletes Against Apartheid. He called U.S. investments in South Africa "Tarzan money," saying dollars "swing in . . . and swing out with profits" made from the blood of oppressed and exploited black workers.

Abourezk, Sturdivant and McManus were arrested after walking up Massachusetts Avenue NW to the embassy, trying unsuccessfully to meet with the ambassador and then holding a sidewalk demonstration. The other nine protesters were taken into custody after they drove the block's distance in two cars and joined the other three in the sidewalk demonstration.