Movement for Democratic Change spokesman Nelson Chamisa told The Telegraph newspaper Tsvangirai was taken into custody near Bulawayo as he campaigned against incumbent President Robert Mugabe in the June 27 run-off election.

Tsvangirai was set free late Wednesday, The New York Times reported.

"It makes absolutely no sense that a presidential candidate in an election is arrested for attracting crowds of people," said Tsvangirai's party in a statement.

Amnesty International condemned the arrest and what it called a "sharp and dangerous crackdown," the Times said.

"It appears they want to disrupt our campaign program," Chamisa told The Telegraph.

He said fair elections in Zimbabwe will be impossible without the intervention of the United Nations and the international community,

The opposition party says 50 of its supporters have been slain by Mugabe's supporters in recent weeks, with soldiers loyal to the president beating and threatening those who voice support for Tsvangirai, The Telegraph said.

Tsvangirai's arrest came hours after it was reported that the Mugabe government accused international charities of engaging in political activities and ordered them to stop operating in Zimbabwe.