Seychelles President Albert Rene has demanded that South Africa return the 45 mercenaries involved in an abortive attempt to overthrow his government last Wednesday as a sign of Pretoria's sincerity that it knew nothing about their expedition.

The mercenaries, who were arrested in the South African port town of Durban early Thursday after hijacking an Air India jet to escape from the Seychelles, have still not been formally charged in court. The statutory limit after which arrested suspects must appear in court expired yesterday.

The South African Cabinet "gave urgent attention" to the circumstances surrounding the attempted coup in its routine weekly meeting today, the state-run television reported.South African Police Minister Louis Le Grange told a television interviewer that the police investigation was completed and "certain decisions" had been taken that would be implemented "in the next few days."

A dissident group from the Seychelles, Le Mouvement Pour la Resistance, has claimed responsibility for the coup attempt. A spokesman for the group, Paul Chow, said in London yesterday that his group, comprised of 100 active members living in Britain, Australia, the United States and Africa, including South Africa, would try to stage another attempt to unseat Rene.

The group has no recognized leader, but former Seychelles president James Mancham said in London he was called and asked to give his support to the coup attempt. Mancham denied any role in plotting the coup but said he would support any group that could unseat Rene, who toppled him in a 1977.

In a national radio address this morning, Rene noted South Africa's denials of involvement in the mercenaries' activities. "If South Africa is speaking the truth then we demand that the mercenaries be returned to the Seychelles to face trial," Rene said.

Seychelles director of information Tonio Beaudoin said in a telephone interview that his government had not sent any formal request through diplomatic channels for the mercenaries' extradition because the two countries do not have diplomatic relations. But he said Rene sent a message to the Organization of African Unity asking for its support for the extradition request.

A 24-hour curfew has been in effect since the coup attempt and among those marooned on the island is the U.S. ambassador to Kenya and the Seychelles, William Harrap, who normally resides in Nairobi. Reached by telephone yesterday at his hotel, Harrap said he arrived in the Seychelles Nov. 22 for a routine visit and had planned to leave last Sunday.