The arrest of a top aide to Prime Minister Morgan Tsvangirai has marred the swearing-in of Zimbabwe's new unity government on Friday.

Despite the arrest of Roy Bennett, a white farmer who became treasurer of Tsvangirai's Movement for Democratic Change (MDC) and was designated to become deputy agriculture minister, Mugabe said he was committed to the new government.

"When I say I am committed I mean it. When I say I want to work with you sincerely and honestly, I mean it," he declared in a speech after presiding over the swearing in ceremony.

"The people will expect a lot from us. Let's never let them down. It should never be forgotten that the suffering of our people is our suffering. Our failure hurts them and our success yields benefits to them."

Bennett, who returned just last month from three years of self-imposed exile in South Africa - where he had fled to escape charges of plotting to kill Mugabe - was arrested at a Harare airport, the MDC said.

"It is very disturbing. I don't understand the rationale," said Innocent Gonese, MDC's chief whip in parliament, who said that a court had already dismissed the same charges against other party members.

"It undermines confidence in the all-inclusive government."

The swearing-in had already been held up by more than two hours as the MDC accused Mugabe of trying to bring 22 ministers into cabinet, although their agreement allowed his ZANU-PF party only 15 seats.

In the end, Mugabe swore in two extra ministers while the MDC took one more seat than expected.

Tsvangirai's party in a statement blamed the development on "back-stage chaos and confusion" within ZANU-PF.

Mugabe gave some of his party's staunchest hardliners the key posts of defence, home affairs and national security.

The defence portfolio went to Emmerson Mnangagwa, seen as the president's right hand, who earned notoriety as the head of state security in the 1980s, when a North Korean-trained army brigade allegedly massacred up to 20 000 suspected dissidents from the minority Ndebele people.

Sydney Sekeramayi, who was defence minister in the last cabinet, took up the state security post.

Kembo Mohadi retained his position at home affairs, a portfolio he will share with a co-minister from Tsvangirai's party.

At home affairs, Mohadi has presided over the police during a period that saw widespread accusations against the force of rights abuses and intimidation of the MDC.

Top among those are allegations that security forces have detained and tortured MDC supporters and other activists at secret camps.

About 30 activists remain in custody, and Bennett's arrest raised new doubts about their fate.

Bennett was among the most striking names on Tsvangirai's cabinet list, which made for an unlikely partnership with some of Mugabe's oldest and toughest allies.

Bennett's Charleswood farm was expropriated under Mugabe's land reforms in 2003, and the following year he was jailed for eight months for assault after he punched the justice minister during a heated debate in parliament on the land programme.

In 2006, he fled to neighbouring South Africa to escape arrest after being implicated in an alleged plot to assassinate Mugabe. Such allegations have been levied but never substantiated against much of the MDC leadership.

Under the unity pact, Tsvangirai's aides are paired with people they have long accused of masterminding political attacks against themselves and their supporters.

Analysts have questioned how such bitter adversaries would be able to work together to curb a humanitarian crisis that almost defies comprehension.

Nearly seven million people need food aid. Up to three million have fled the country. Unemployment is at 94 percent and only 20 percent of children are going to school.

Public hospitals are closed, even though 1.3 million people have HIV. A cholera epidemic is ravaging the country, hitting about 70 000 people and killing 3400 since August.

Sapa