HARARE (Reuters) - Zimbabwe police arrested opposition leader Morgan Tsvangirai on Thursday while he was campaigning for the country's June 27 presidential run-off election, his Movement for Democratic Change (MDC) said.

It is the third time in little more than a week that Tsvangirai has been detained by police.

The opposition and human rights groups accuse President Robert Mugabe's government of waging a campaign of arrests and violent intimidation ahead of the run-off with Tsvangirai.

"Morgan Tsvangirai has been arrested by Zimbabwean police and is currently being held at Kwekwe police station. 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.

Police earlier on Thursday arrested the party's secretary general at Harare airport as he flew in from South Africa. Biti, the party's number three, left the country soon after disputed March 29 elections to gather African support.

"He was wanted in connection with the premature announcement of results before the official announcement of results by the Zimbabwe Electoral Commission," said police spokesman Wayne Bvudzijena.

Tsvangirai beat Mugabe in the first round of elections in March but not by enough to avoid a run-off, according to official figures.

The MDC says ZANU-PF activists have killed 66 opposition supporters to try to intimidate voters before the run-off, and police have detained Tsvangirai twice over the past week while trying to campaign.

The ruling party blames the opposition for the political violence.