Secretary-General Tendai Biti flew home from South Africa ahead of a June 27 presidential election run-off between Tsvangirai and President Robert Mugabe, who is battling to keep his 28-year hold on power in the ruined state.

Police had sought Biti, the third highest ranking official in the Movement for Democratic Change, 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. "He is in police custody and we are still investigating the matter."

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 a 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 the once prosperous country.

Tsvangirai was detained for about two hours as he was campaigning in Kwekwe, 200 km (125 miles) west of Harare. He resumed his campaign after he was released.

"This is just a deliberate attempt to delay us, it is just harassment," Tsvangirai's spokesman George Sibotshiwe said.

Biti had left Zimbabwe shortly after the poll to galvanise support for the opposition within Africa. He said earlier this week that he was returning to Zimbabwe to help build democracy.

BLOODSHED

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.

Some 400 SADC observers are expected across Zimbabwe in the next two weeks.

"It is a mammoth task," Tanki Mothae, one of the members of the observer mission, said at a press conference in Harare. "This is to help the people of Zimbabwe go through this election as peacefully as possible."

The political turmoil has compounded an economic crisis.

Inflation has soared to over 165,000 percent, unemployment hovers 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. Some of the most fertile farms have gone to supporters who were ill-equipped for agriculture.

The official Herald newspaper quoted Mugabe as saying only 42 percent of land was being tilled completely and renewing threats to re-possess farms that were not being properly used.