The secretary general of Zimbabwe's main opposition party will be charged with treason and faces a possible death sentence, police say.
Tendai Biti of the Movement for Democratic Change (MDC) was arrested in Harare on his return from South Africa.
MDC leader Morgan Tsvangirai, who faces President Robert Mugabe in a 27 June run-off poll, was also detained twice.
Meanwhile, the BBC has obtained evidence suggesting the military is involved in the Mugabe poll campaign.
The documents outline plans by the ruling Zanu-PF party to harass and drive out opposition supporters, especially from rural areas.
Thursday also saw the US accuse Zimbabwe's security forces of seizing a lorry loaded with American food aid for schoolchildren, and distribute it to ruling party members.
National police spokesman Wayne Bvudzijena said Mr Biti would be charged with treason "for publishing a document that was explaining a transitional strategy around March 26".
Morgan Tsvangirai faces a run-off with Robert Mugabe on 27 June
He said he would also be charged for proclaiming victory in the 29 March elections before official results were published.
"For the treason charge he faces the death penalty or life in prison," he said.
"He is in police custody and we are still investigating the matter."
Zimbabwe's deputy Information Minister Bright Matonga told the BBC's Focus on Africa that Mr Biti was due to appear in court soon.
Mr Biti had been aware he faced arrest once he returned from South Africa, where he had been 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.
Mr Tsvangirai was detained twice on Thursday - the first time at a roadblock near the central town of Kwekwe on the way to an election rally.
He was released after two hours only to be picked up later by a different group of police officers and detained for several hours. He has now been held four times this month.
"It's just harassment, but we will be continuing with our campaign tomorrow," said Mr Tsvangirai's spokesman George Sibotshiwe, who was arrested with the MDC leader.
The US ambassador to Zimbabwe, James McGee, said Washington was "very, very concerned" about the treason charge against Mr Biti.
The BBC's Ian Pannell finds evidence that the army is involved in violence
He said he had seen the document the charge was based on, and described it as a routine policy plan of the kind any political party would write ahead of an election.
The lorry of American food aid, US officials said, was seized about 6 June.
"We have information that the humanitarian food was then distributed to government party members at a government party rally," US state department spokesman Gonzalo Gallegos said.
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 Tsvangirai said this week that Zimbabwe was "effectively being run by a military junta".
He said more than 60 opposition supporters had been killed in political violence since the March elections and 200 more were unaccounted for.
Mr Mugabe's supporters say the scale of the violence has been exaggerated and blame the MDC for some attacks.
But human rights groups accuse the ruling party of being behind most of the violence.