Movement for Democratic Change leader Morgan Tsvangirai was jailed for four hours in what a party spokesman on Saturday called an act of harassment by security forces.
Police stopped Tsvangirai at a roadblock Friday after he held a campaign rally at Chiredzi, 300 miles south of Harare. Tsvangirai was released after midnight, party spokesman David Coltart said.
Two lower-ranking party officials who were traveling with Tsvangirai were also arrested, as were two bodyguards. They were held overnight and released early Saturday afternoon.
But Coltart said "Arrests late at night are deliberate harassment of senior members of an opposition political party." He added the group was not informed of charges against them.
Police were not immediately available for comment.
Since the Movement for Democratic Change led the defeat of President Robert Mugabe's ruling party in a referendum on a revised constitution in February, violent mobs and ruling party militants have staked claims to land on more than 1,000 farms, many of them owned by white opposition supporters.
Mugabe has called the land occupations justified in this former British colony where one-third of the fertile land is still owned by about 4,000 whites. But opponents accuse Mugabe of provoking the land crisis to intimidate white farmers and other opposition supporters in advance of parliamentary elections.
In the worst economic crisis since independence, soaring inflation and unemployment and gas shortages have fueled support for the main labor-backed opposition group ahead of the elections, expected to be called within months. Tsvangirai's party is seen as the biggest threat to Mugabe's hold on power since the president led the nation to independence from Britain in 1980.
Political violence has claimed at least 17 lives in recent weeks. Farmers' leaders on Friday reported worsening intimidation against workers on white-owned farms.
The once-thriving Zimbabwe International Trade Fair was dotted with vacant stalls Saturday, another sign of the country's tottering economy and political turmoil. There were only 617 exhibitors, down from 820 last year. Most foreign exhibitors were not present.
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