A powerful bomb exploded early Wednesday at the headquarters of South Africa's Council of Churches and other anti-apartheid movements, injuring 21 people and causing extensive damage along an entire city block, police said.

Anglican Archbishop Desmond Tutu condemned the "evil" blast as the work of "perpetrators or supporters of apartheid," and a spokeswoman for the Black Sash women's movement with offices in the six-story building branded the attack "an attempt to silence us."Most of the victims, including 19 blacks, a white woman and a white child, were injured as the windows of their apartments across the street shattered from the force of the explosion that occurred around 1:20 a.m., police spokesman Capt. Pierre Louw said.

The blast also ignited gas leaking from damaged mains at the headquarters of the SACC and other social and community movements opposed to the white-led minority government, and city officials said the building was likely to be declared unsafe for occupation.

Police and witnesses said the floor of the entrance lobby collapsed into the parking basement.