South African police today continued their latest roundup of opposition leaders, with 10 more arrests, while the country's top law-and-order official issued a stern warning against a mass demonstration planned for Wednesday to demand the release of imprisoned black nationalist Nelson Mandela.

The warning set the stage for a new confrontation between opponents of white-minority rule and police that could set off another round of violence that has already claimed more than 625 lives in the past year. One more victim was added to the death toll today as police reported incidents in nearly 20 black townships around the country.

The arrests of 10 activists, some black and others white, as announced by police today, brought to 27 the total picked up over the last two days in Cape Town, Durban and Johannesburg. The group includes trade unionists, clerics, lawyers and academics, all of them associated with the multiracial United Democratic Front, a leading opposition movement inside South Africa battling the apartheid system of race segregation.

Their houses were searched, as were the front's Cape Town offices, and family members said large stacks of papers, books and other records were confiscated.

Those arrested are being held under a section of South Africa's stringent internal security law, which provides for indefinite imprisonment in solitary confinement.The new arrests, a front spokesman said, were "no doubt" connected to the planned protest of the imprisonment of Mandela, a 67-year-old black resistance leader who is serving a life sentence for treason.

Law and Order Minister Louis le Grange asked the public to ignore the call for the mass protest, saying it would violate the government's ban on outdoor political gatherings.

"No illegal gathering will be allowed and the police will take stern action in this regard," he warned.