President Robert Mugabe's supporters have used violence to intimidate opponents before next week's Zimbabwe election, Human Rights Watch said on Wednesday.

However, the head of a Southern African Development Community (SADC) observer mission said it believed the political environment was conducive to a free election.

Mugabe faces the strongest challenge to his 28-year rule in presidential, parliamentary and municipal elections on March 29 because of defections by senior officials of the ruling ZANU-PF party and a deepening economic crisis.

"As in previous elections, local government authorities, ZANU-PF supporters, and security forces including the police and central intelligence, are the main perpetrators of the violations ...," Human Rights Watch, a U.S.-based rights group, said in a report released in Johannesburg.

Opposition groups have accused Mugabe, who has ruled since independence from Britain in 1980, of rigging previous elections, allegations he denies.

"Despite some improvements on paper to the election regulations, Zimbabweans aren't free to vote for the candidates of their choice," said Georgette Gagnon, Africa director of Human Rights Watch.

"While there are four candidates running for president and many political parties involved, the election process itself is skewed."

The head of the SADC mission said the voters' register was published late "and there could be other irregularities".

"SADC does not operate on the principle of all-or-nothing basis because no election process can ever be perfect," Jose Marcos Barrica told a news conference.

CRACKDOWN WARNING

Barrica said his SADC team did not believe that statements by two senior security officials they would not accept an opposition victory represented the official position.

The statements have generated controversy in a largely peaceful election campaign for the March 29 votes. Mugabe hopes to fend off challenges from long-time rival Morgan Tsvangirai, leader of the biggest faction of the opposition Movement for Democratic Change (MDC), and former finance minister Simba Makoni, who was expelled from ZANU-PF.

Zimbabweans are suffering from the world's highest inflation rate -- officially put at more than 100,000 percent -- and chronic shortages of food, fuel and foreign currency.

The government has used state-subsidised food and farming equipment as a tool to gain political advantage, Human Rights Watch said.

Prices of some basic goods, including the staple maize meal, bread, cooking oil and soap, have risen by up to 300 percent since the start of this month.

Mugabe blames the country's economic troubles on Western foes, especially former colonial master Britain.

Addressing a rally in Chinhoyi, west of Harare, on Wednesday, Mugabe gave businesses a one-week ultimatum to reduce prices or face a government crackdown.

"Those who have raised prices must bring them down quickly. Otherwise we will bring both the prices and those profiteering down," Mugabe told supporters in a packed stadium.