Tendai Biti regularly stands in MDC leader Morgan Tsvangirai
Police in Zimbabwe have arrested the leader and secretary general of the opposition Movement for Democratic Change ahead of the 27 June run-off.
Party head Morgan Tsvangirai was detained while campaigning near the central town of Kwekwe.
Police held Secretary General Tendai Biti on his return from South Africa.
The BBC has obtained documents suggesting the military is actively involved in running Robert Mugabe's campaign for re-election.
Mr Tsvangirai was detained at a roadblock with his entourage on the way to an election rally, and was being held at Kwekwe police station, the MDC said.
It is the third time he has been arrested this month.
Mr Biti was detained at Harare international airport, the party said in a brief statement, giving no details of any charges laid against him.
He has often deputised for Mr Tsvangirai, who faces President Robert Mugabe in a run-off vote on 27 June.
Mr Biti had been aware he faced arrest once he returned from South Africa.
Meanwhile, five people have been killed in political violence in the southern Zimbabwean town of Masvingo in less than a week, BBC contributor Owen Chikari reports.
At least four of those killed were MDC supporters, party official Wilstaf Stemele said.
Mr Biti had been in South Africa for two months.
Since the first round of the presidential election on 29 March, he has regularly hosted news conferences and acted as a deputy to Mr Tsvangirai.
Several weeks ago, the Zimbabwean authorities indicated they wanted to question him.
Before flying from Johannesburg to Harare, he said that, notwithstanding the threats of arrest, he was going home to be part of the MDC leadership in the country.
He told reporters that returning under threat of arrest was a "stupid decision" but he believed he had to return to continue the battle for change.
"The only crime I have committed is fighting for democracy," he said before hugging an aide and disappearing through the boarding gate at Johannesburg's OR Tambo International Airport, the Associated Press reports.
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