Senior MDC member Roy Bennett was later released on bail after two days in prison, but Tsvangarai's spokesman insisted the boycott would continue.

Bennett was detained for alleged arms and terrorism offences.

Tsvangirai said the detention showed Zanu-PF was an "unreliable" partner.

"It has brought home the reality that as a movement we have an unreliable and unrepentant partner in the transitional government," AFP quotes him as saying.
 
Mugabe and Tsvangirai agreed to share power in February after disputed elections last year marred by violence.

But since then their parties have failed to agree on appointees for provincial governors, the central bank governor and the attorney general.

The Movement for Democratic Change also accuses Zanu-PF of persecuting its legislators and activists.

Zimbabwean journalist Brian Hungwe told the BBC Mr Tsvangirai said the MDC was not yet pulling out of the coalition government and was open to talks to break the impasse.

But the prime minister also accused Zanu-PF of beginning to set up militia bases in the countryside, and militarising state institutions ahead of future elections.