Fire gutted the home Thursday of Winnie Mandela and jailed black guerrilla leader Nelson Mandela, and police said it was set by a group of black teenagers.

No one was home when the blaze broke out at about noon, and there were no injuries, police and witnesses said.Mandela arrived by car about an hour later but remained in her vehicle and drove to her nearby office without making any statements.

The four-room home in the black township of Soweto outside Johannesburg was charred, and all the contents were destroyed in the fire, officials said.

Mandela, a social worker and anti-apartheid activist, shared the modest home with her daughter Zindzi, 27, and two grandchildren. The house belonged to Nelson Mandela before the couple was married 30 years ago.

Police Capt. Ruben Bloomberg said in Pretoria that a group of black high school students stoned the Mandela home, then doused it with gasoline and set it alight.

He said he did not know how many youths were involved nor the motive for the attack. No arrests have been made, he said.

Teenage boys known as "comrades," who regularly guard the house, pushed journalists away from Mrs. Mandela's office after the fire, and one threatened to burn a photographer's car.

The teenagers blamed the fire on students from a high school in Soweto. They refused to say anything more about the incident.