"We were arrested as the campaign bus was approaching Kwekwe," George Sibotshiwe told AFP news agency by phone from the scene.
"We are now being taken to Kwekwe police station."
State televsion reported in April that Biti was being investigated on suspicion of being the author of a plot to rig the outcome of the March 29 presidential and parliamentary elections.
"He will be charged with contravening section 20 of the criminal law codification for publishing a document that was explaining a transitional strategy around March 26 which in its case is a treasonous charge," Wayne Bvudzijena, national police spokesman, said.
Police had also been seeking his arrest after he announced the MDC had won the general elections before the official results were released.
Biti left the country shortly afterwards.
According to MDC officials and witnesses, Biti was arrested in a tunnel leading from his plane to the immigration hall by a group of 10 plain-clothes officers and without having any contact with his lawyers.
"They handcuffed and took him to a room downstairs," Luke Tamborinyoka, the MDC's director of information who was at the airport, told the AFP news agency.
Tsvangirai officially fell just short of an outright majority in the first round but insists he in fact won more than 50 per cent and is only taking part in a run-off vote under protest.
He has faced major obstacles to his campaign, with a series of MDC rallies banned as well as the previous detentions.
The opposition says more than 60 of its supporters have been killed since the initial polling as part of an organised campaign of intimidation.
Mugabe has banned Western observers from overseeing the polls but he has authorised African monitors to attend the second round, including a team from the 14-nation Southern African Development Community (SADC).