Police used tear gas and clubs Tuesday against activists supporting a boycott of Wednesday's nationwide elections for segregated municipal councils. Archbishop Desmond Tutu called for an election day fast and appealed to all sides to avoid violence.

A bomb exploded Tuesday in Potchefstroom, southwest of Johannesburg, extensively damaging a shopping center and slightly injuring a policeman, police headquarters said.It was the second bombing in as many days and police said the attacks are part of a campaign by the African National Congress black guerrilla movement to disrupt the elections. On Monday, a car bombing in Witbank, east of Johannesburg, killed two people and injured 42.

In Zwide, near Port Elizabeth, a black candidate was shot and killed at his home late Monday by an unidentified assailant, a local newspaper reported.

Near Cape Town, more than 30 mixed-race high school students were injured today in a clash with club-wielding police after they protested the elections, a school official said.

At Johannesburg's University of the Witwatersrand, more than 1,000 people staged an anti-election rally. Many of them, including black activist Winnie Mandela, marched to a campus gate where riot police ordered them to disperse, then fired tear gas.