Zimbabwean police today stepped up their campaign of arrests against the opposition, detaining its leader and announcing plans to charge the secretary-general with treason.

Opposition officials said Morgan Tsvangirai, head of the Movement for Democratic Change, was arrested at a roadblock while campaigning in Kwete, a town 125 miles of the capital Harare. Secretary-General Tendai Biti, who could face the death penalty if convicted, was seized at Harare airport as he flew in from South Africa.

“We are charging him with treason and communicating statements prejudicial to the state,” said Wayne Bvudzijena, police spokesman.

“For the treason charge he faces the death penalty or life in prison. He is in police custody and we are still investigating the matter."

Mr Biti faces the treason charge for what was described as a transition document discussing changing Zimbabwe’s government. The second charge relates to accusations that he announced election results before the official count was released.

The MDC declared Mr Tsvangirai the winner of the presidential election based on its own counts of results displayed at polling stations, fearing that Mr Mugabe would otherwise use the electoral commission to steal the vote. According to official figures, Mr Tsvangirai beat Mr Mugabe in the first round of elections in March but did not get the majority needed to avoid a run-off.

For Mr Tsvangirai, it was the third time in little more than a week that he had been detained by police. After two hours' interrogation, he was released without charge.

The opposition and human rights groups accuse President Robert Mugabe’s government of waging a campaign of arrests and violent intimidation ahead of the scheduled presidential run-off with Mr Tsvangirai, set for 27 June.

“He and his entourage of 20 people were detained at a roadblock outside of Kwekwe while conducting a campaign tour through Midlands province,” the MDC said in a statement.

In a telephone call from the police station, Mr Tsvangirai’s spokesman, George Sibotshiwe, said police had told them they wanted to search for something.

He added: “This is just a deliberate attempt to delay us, it is just harassment.”

On the phone, Mr Tsvangirai could be heard asking a police officer what crime he had committed and saying that the bus, on which a large poster of the MDC leader’s face was displayed, was part of the opposition campaign strategy.

Mr Biti, the party’s number three, left the country soon after disputed March 29 elections to gather African support. He has held sporadic talks with Mr Mugabe and his arrest may signal the president’s rejection of any negotiated solution to his country’s country political and economic crisis.

Mr Biti recently said that returning to the country under threat of arrest was “a stupid decision”. But he added that he felt compelled to continue the battle for change.

“The only crime I have committed is fighting for democracy,” he said at the Johannesburg airport.

The MDC says Zanu-PF activists have killed 66 opposition supporters in a bid to intimidate voters before the run-off. Mr Mugabe's militia have also been accused of burning family members of opponents alive, including the six-year-old son of an elected MDC councillor. The ruling party blames the opposition for the political violence.

*UN Secretary Ban Ki Moon is sending Haile Menkerios, a UN diplomat and former Eritrean ambassador, to Zimbabwe from 16-20 June “for discussions on the political situation and the upcoming elections”.

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