Zimbabwe's security forces have been put on full alert to quash any violence at Saturday's general election and will not let candidates declare victory before official results, the police chief said on Friday.

President Robert Mugabe faces his toughest challenge of 28 years of power in the elections, held amid a desperate economic crisis. Some security chiefs have said they would not accept an opposition victory and Mugabe's rivals have accused him of using the forces to try to rig the vote.

Police chief Augustine Chihuri told a news conference that security forces would do their duty without fear or favour.

"We wish to advise the nation that all the defence and security forces of Zimbabwe are on full alert from now onwards covering the election period and beyond," he said.

"We are not going to allow a situation where individuals arrogate themselves winners at any stage of the electoral process," said Chihuri, flanked by the chiefs of all of the country's army and security services.

Zimbabwe holds joint presidential, parliamentary and council elections on March 29.

In the presidential election, Mugabe faces his former finance minister Simba Makoni, who defected from the ruling ZANU-PF party, and old rival Morgan Tsvangirai, leader of the largest faction of the Movement for Democratic Change.