Zimbabwe's election body has no legal powers to stop security chiefs from threatening to reject an opposition victory in this month's poll, a senior official said on Tuesday.

Analysts say President Robert Mugabe faces the strongest challenge to his 28-year rule in presidential, parliamentary and municipal elections on March 29 due to an economic meltdown and and a pair of opposition candidates.

Statements by two senior security officials that they would only welcome a victory by Mugabe and his ruling ZANU-PF party have generated controversy in a largely peaceful campaign ahead of the election.

Responding to opposition complaints at a meeting in Harare on Tuesday, Zimbabwe Electoral Commission chairman George Chiweshe said there was no law mandating the commission to act against those making the threats.

"The commission will not take a populist stance and do things or make pronouncements which are not in its mandate. There is nothing in the law allowing us to take a position on this," he said, drawing laughter from diplomats, local and regional election observers and journalists.

Mugabe hopes to fend off challenges from long-time rival Morgan Tsvangirai, leader of the larger faction of the Movement for Democratic Change (MDC), and former finance minister Simba Makoni.

South Africa's ruling African National Congress said on Monday security forces should stay out of the election.

Chiweshe said the commission would be impartial during the polls.

State media reported on Tuesday that Mugabe had invoked presidential powers to change the law to allow police into polling stations to assist illiterate or disabled voters.

Last Friday, police commissioner Augustine Chihuri vowed he would not allow "western-backed puppets" to rule Zimbabwe, repeating similar comments made last month by the head of the prison service, retired army major-general Paradzayi Zimondi.

In 2002, Zimbabwe's security chiefs made a controversial statement suggesting they would not accept a victory by Tsvangirai. Mugabe narrowly won the presidential election amid opposition charges of rigging.

Chiweshe said the commission was well prepared for this month's poll, which opposition groups and some western nations have said was unlikely to be free and fair.

Mugabe's government has barred observers from the European Union, which is locked in a bitter stand-off with Harare over accusations of human rights abuses and economic mismanagement.