(Adds Tsvangirai's release, reaction from Britain and U.S.)

By MacDonald Dzirutwe

HARARE, June 12 (Reuters) - Zimbabwean police arrested opposition leader Morgan Tsvangirai twice more on Thursday in what his spokesman said was harassment aimed at disrupting his campaign for a June 27 run-off election.

Tsvangirai's latest arrest, at a roadblock near Gweru, outside Harare, came hours after police said his party's Secretary General Tendai Biti would be charged with treason and could face the death penalty. Biti was arrested on Thursday.

Tsvangirai was later released without charge.

"We have just been released, some five minutes ago. We were not charged but held at Gweru Central Police Station for about three hours," said Tsvangirai's spokesman George Sibotshiwe, who was among those arrested with the MDC leader.

"Our vehicles were searched. It's just harassment, but we will be continuing with our campaign tomorrow," he said.

It was the fourth time in about a week that Tsvangirai had been detained. He was held for two hours earlier on Thursday.

Biti was in police custody after being arrested at the airport in Harare. He flew home from South Africa to help Tsvangirai's campaign against President Robert Mugabe, who is battling to keep his 28-year hold on power in the ruined state.

Police had sought Biti, the third ranking MDC official, for announcing results of the March 29 first round vote prematurely.

"We are charging him with treason and communicating statements prejudicial to the state. For the treason charge he faces the death penalty or life in prison," police spokesman Wayne Bvudzijena said.

The MDC described the charges as ludicrous and demanded that defence lawyers be allowed to see Biti.

Britain and the United States expressed concern at Biti's arrest. "We and the rest of the international community are watching and we hold the Zimbabwe government responsible for his physical safety," British Foreign Secretary David Miliband said.

INTIMIDATION CAMPAIGN

The opposition and human rights groups accuse Mugabe's supporters, including the security forces, of arresting and attacking its opponents in a bid to intimidate the opposition ahead of the run-off.

Tsvangirai defeated Mugabe in the March 29 election but failed to win the absolute majority needed to avoid a second ballot, according to official results. Mugabe's support has been eroded by the economic collapse of his once prosperous country.

The MDC says ruling ZANU-PF party activists have killed 66 opposition supporters since March. Mugabe and his officials blame the opposition for the violence.

The Southern African Development Community, a grouping of 14 nations including Zimbabwe, has sent a team of election monitors. Observers from Western nations critical of Mugabe's government are not being allowed into the country.

The political turmoil has compounded an economic crisis.

Inflation has soared to over 165,000 percent, unemployment is around 80 percent and food and fuel shortages are commonplace. Millions have fled to neighbouring countries in search of food and work.

Critics blame Zimbabwe's decline on Mugabe's policies, including the seizure of thousands of white-owned farms which they say has contributed to the collapse of agriculture since 2000. Some of the most fertile farms have gone to supporters of Mugabe who were ill-equipped for farming. (Additional reporting by Cris Chinaka; writing by Marius Bosch and Paul Simao; editing by Andrew Roche)

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