The cardinals of the Roman Catholic Church have voted to start their conclave to pick a successor to the late Pope Paul VI on Aug. 25, the Vatican announced yesterday.

The site of the meeting was not immediately announced but Vatican sources said it will most likely be the Sistine Chapel, where must such conclaves have been held in the past.

Aug. 25 will be 19 days after Pope Paul's death last Sunday following a heart attack at his summer palace in Castel Gandolfo. Under church law, a conclave must be started between 15 and 20 days after the death of a pope. tr for add one.

Meanwhile, more than 60,000 mourners filed past the Pope's body yesterday as it lay on a low catafalque in front of the great altar of St. Peter's Basilica. The crowds, moving 12 abreast, went past the body at a rate of 200 per minute.

The body is to be on display for two days with burial Saturday. Vatican officials initially said the coffin would be sealed after it was brought from Castel Gandolfo, because the body was decomposing. They reversed themselves after cardinals and other prelates arriving from abroad asked that it be opened so they could pay homage.

The body was showing further signs of decomposition, however, and Vatican officals said the decision to keep it exposed would be reviewed hourly.

Among those who joined in the public homage yesterday was William Cardinal Baum of Washington.

The White House announced other members of the American delegation to the funeral that will be led by President Carter's wife, Rosalynn. She will be accompanied by Sen. Edward M. Kennedy (D-Mass.), Gov. Hugh Carey of New York, Rep. Robert N. Gialmo (D-Conn.) David Walters, the president's personal representative to the Vatican, and Leonel J. Castillo, commissioner of immigration and naturalization.