Mr Bennett, here with PM Tsvangirai, had only just returned to Zimbabwe
Police in Zimbabwe have charged a senior member of Prime Minister Morgan Tsvangirai's MDC party with treason, the party says.
Roy Bennett, nominee for deputy minister of agriculture, was charged just hours after the new power-sharing cabinet was sworn in, the MDC says.
The party's statement described the charges against him as "scandalous" and "politically motivated".
Earlier, the ceremony was marred by a row over the allocation of ministers.
Under a power-sharing deal agreed after months of talks following disputed elections, President Robert Mugabe's Zanu-PF is to have 15 posts and the two factions of the MDC 16 posts in the government.
But seven extra Zanu-PF members turned up at State House in the capital, Harare, to be sworn into office. The issue was only resolved after intense closed-door negotiations.
Correspondents described the day as an extremely bumpy start to Zimbabwe's power-sharing experiment.
The MDC denounced the accusations against Mr Bennett.
"These charges are scandalous, vexatious and without basis in law, but are simply politically motivated, simply intended to justify the continued incarceration of Roy Bennett," the party said.
The former MP, seized near a Harare airport earlier in the day, has long been a controversial figure.
KEY CABINET APPOINTMENTS
Emmerson Mnangagwa, seen as a possible successor to Robert Mugabe, defence
Sydney Sekeramayi, ex-defence minister, state security
Joseph Made, ex-agricultural minister, agriculture
Patrick Chinamasa, chief negotiator of power-sharing deal, justice
Kembo Mohadi, former home affairs minister, home affairs
Tendai Biti, lawyer and ex-student leader, finance
Giles Mutsekwa, lawyer with a military background, home affairs
Henry Madzorera, trained doctor, health
Eric Matinenga, lawyer who defended Mr Tsvangirai on treason charges, constitutional affairs
Roy Bennett, former farmer, arrested, deputy agriculture minister
A white farmer who lost his property under Mr Mugabe's land reform programme, he was in prison from October 2004 to June 2005.
The sentence was imposed by other MPs after he pushed a minister during an argument in parliament over land reform.
He has only recently returned to Zimbabwe after more than two years in South Africa, where he fled after police sought his arrest in connection with an alleged plot to kill Mr Mugabe.
Mr Bennett was taken to a police station in the eastern city of Mutare, where police later reportedly fired shots into the air to disperse MDC supporters.
The MDC called for his immediate release.
Correspondents say Friday's hitches show how difficult it will be for the coalition - agreed after strong international pressure and amid the collapse of the country's economy - to work.
Many of the Zanu-PF ministers have served in cabinet since Mr Mugabe was first elected in 1980. Several MDC ministers have been beaten or arrested for their opposition to Mr Mugabe.
But in an interview with the BBC before the cabinet was sworn in, Mr Tsvangirai said that the two sides had to work together.
"Mugabe may be part of the problem, but he's also part of the solution. I am sure the reverse will also apply to me from their side," he said.
"But we have reached the stage now where we say we have to have a negotiated settlement, for the sake of the people".