"Police have again changed charges on Roy Bennett. They have now charged him with treason," the party said in a statement.
"These charges are scandalous, vexatious and without bases in law, but are simply politically motivated, simply intended to justify the continued incarceration of Roy Bennett."
Police earlier fired live ammunition in the air to disperse opposition MDC supporters who had surrounded the police station in Mutare in the east of the country, the MDC said.
Bennett was nominated by Prime Minister Morgan Tsvangirai as deputy minister of agriculture.
While the MDC said Bennett must be immediately released unconditionally and unharmed, it did not threaten any action that could endanger the new unity government.
Hours earlier, Mr Tsvangirai, the new Prime Minister, had blithely dismissed Western alarm about his decision to enter the unity Government with Mr Mugabe. "You are too paranoid about Mugabe," he told The Times in an exclusive interview.
Tonight, however, Western diplomats said that the day’s events had confirmed their worst fears. "This is not power-sharing, it’s a power struggle," one said. "Someone is saying, ‘We’re still boss. It’s our country. We don’t have to make concessions’."
The drama began when Roy Bennett, a popular former MP in the Movement for Democratic Change (MDC) who is due to become Deputy Agriculture Minister, was seized after secret police stopped his charter aircraft as it taxied at a small Harare airport.
Mr Bennett, 52, who was flying to Johannesburg to spend the weekend with his wife, was bundled into a car and driven to a police station 25 miles away, followed by MDC security men.
Mr Bennett was later taken to the eastern city of Mutare where, according to the MDC, he was charged with treason. "These charges are scandalous, vexatious and without basis in law," the MDC said in a statement.
Police fired shots in the air to disperse hundreds of opposition supporters who had gathered outside the police station where he was being held.
Mr Bennett is a former coffee farmer whose land was seized in 2003. He fled to South Africa in 2005 when the intelligence services tried to arrest him over charges of plotting to assassinate Mr Mugabe.
He returned two weeks ago to endorse Mr Tsvangirai’s decision to enter a unity Government, and the secret police had been hunting him for the past few days.
As Mr Bennett was being arrested, Mr Mugabe arrived at State House in Harare to swear in the Cabinet, clutching a list of 21 Zanu (PF) ministers — seven more than agreed by southern Africa leaders. Frantic haggling ensued, with President Motlanthe of South Africa trying to mediate.
There were reports of fistfights in the Zanu (PF) camp, with politicians who have shamelessly enriched themselves in office refusing to give up their posts. In the end, Zanu (PF) gained two ministers and the MDC one, boosting the Cabinet from 31 to 34 members. The ceremony started two hours late — but, somehow, 36 ministers were eventually sworn in.
Almost all the new Zanu (PF) ministers are hardliners with records of brutality and corruption, and no demonstrable interest in reconciliation with a party that they sought ruthlessly to crush over the past decade. Emmerson Mnangagwa, who helped to orchestrate the slaughter of 20,000 civilians in Matabeleland in the 1980s, got the defence portfolio; Sydney Sekeramayi, the former Defence Minister who set the army on MDC activists during last year’s elections, is in charge of state security. Kembo Mohadi remains the Home Affairs Minister in charge of the police — a force he has turned into an instrument of repression.
In his interview, Mr Tsvangirai acknowledged "residual resistance" to the new Government from Zanu (PF) hardliners and military chiefs but insisted that there was a "high degree of confidence building up" between himself and Mr Mugabe, and that Zimbabwe’s new course was "irreversible".
He appealed to the three million educated Zimbabweans who have fled their country to consider returning. "It is the duty and responsibility of every Zimbabwean to contribute to the rehabilitation and reconstruction of the country," he said. "This \ should inspire Zimbabweans to come home."
Observers, however, saw the day’s events as evidence that Zanu (PF) is deeply divided over sharing power with the MDC. They believe that there is a group of hardliners and generals within Mr Mugabe’s party who are doing their utmost to derail the unity Government. They saw the group’s hand in Mr Bennett’s arrest and in the continued detention of 16 political activists held without charge for months.
At the same time, analysts believe that the debacle over ministerial posts showed the eruption of hostilities within Zanu (PF) as the party loses half its ministerial posts — and all the perks, privilege and patronage that went with them. "It’s going to be huge," said one MDC senator. "You get hard currency at preferential rates, farms, luxury cars . . . and there’s a whole range of perks we don’t know about."
Diplomats believe, increasingly, that Mr Mugabe is losing control of his party. "Mugabe is more and more out of this," said one. Zanu (PF)’s infighting does not help Mr Tsvangirai as he tries to create a stable government capable of tackling the hunger, disease and economic collapse ravaging Zimbabwe. To do that, he told The Times, he needs about $100 million a month from the West — money that is "essential to kick-start the emergency programme we have designed".
After today’s pandemonium, Western officials said that their scepticism had been amply vindicated. "It’s very hard to see any cause for optimism," said one. "This is not a unity Government . . . we’re seeing Zanu (PF) red in tooth and claw."