Zimbabwean security agents had earlier arrested Roy Bennett ahead of a swearing-in ceremony for a new unity cabinet in which he was due to take a post, the party said. There was no immediate comment from police.
The arrest is likely to increase tensions between President Robert Mugabe and new Prime Minister Morgan Tsvangirai, from the MDC, after they ended months of deadlock over a power-sharing deal designed to rescue their ruined country.
The Movement for Democratic Change said in a statement that Roy Bennett, nominated by Tsvangirai as deputy minister of agriculture, had been arrested at the airport and was held at a police station in Mutare in the east of the country.
"Police have started firing live ammunition in the air and have brought dogs in an attempt to disperse hundreds of MDC supporters that had surrounded Mutare police station in support and demanding the release of Roy Bennett," said the MDC.
"Police intend to remove Roy Bennett from the police station to a place they have refused to disclose."
The MDC said police had charged Bennett with trying to leave the country illegally but later gave conflicting information on his case.
Bennett has been living in exile in South Africa after fleeing the country about two years ago because police wanted to question him in connection with the discovery of an arms cache in eastern Zimbabwe.
Foreign investors and Western donors want concrete signs of stability in Zimbabwe. They have made it clear that funds will not flow to the southern African country until a democratic government is created and economic reforms are made.