The leader of Zimbabwe's main opposition party, Morgan Tsvangirai, has been detained and an activist shot dead as they tried to hold a rally.
Mr Tsvangirai leads the Movement for Democratic Change
Mr Tsvangirai and five senior colleagues were seized after trying to hold a meeting that the government said breached a ban on political gatherings.
Riot police used tear gas and water cannons as they fought battles with activists, opposition officials said.
The man died after being shot during the clashes, a police spokesman said.
The spokesman said warning shots were fired after the group defied orders to disperse.
Three police officers were injured and were being treated in hospital, he said.
The incidents occurred at rally called by the Save Zimbabwe Campaign - a coalition of groups agitating for political change.
Mr Tsvangirai's Movement for Democratic Change (MDC) condemned the killing.
"Police are now using live ammunition on innocent, unarmed people and a party activist, Gift Tandare, was the earliest casualty," the party said in a statement.
Officials for the MDC told the BBC that Mr Tsvangirai was being held with five other senior members of the leadership at Highfield police station.
MDC spokesman Eliphas Mukonoweshuro said the protesters were not doing anything wrong.
"It was not a political rally, therefore it was not subject to the provisions of the public order and security act, and there was no permission required to be obtained from the police," he told the BBC.
"But the police went ahead and arrested a broad cross section of leaders of civic organisations, political parties, labour and students."
The three-month ban on political meetings was imposed after violence at an opposition rally last month.
A Save Zimbabwe Campaign statement said lawyers were being denied access to detained supporters. It also accused police of forcing shops, bars and churches to shut down for the day.
"Highfield has been turned into a war zone," it said.
Robert Mugabe, who has led Zimbabwe since independence in 1980, retains an iron grip on power.
1952: Born in Gutu, central Zimbabwe
Left school early to seek work
1974: Started working in a mine
1988: Secretary General on the ZCTU
1997: Organised anti-government strikes
1999: Helped form MDC
2000: MDC won 57 parliamentary seats
2000: Charged with treason, later dismissed
2002: Lost elections to Mugabe, charged with treason
2003: Charged with treason
Last month, in an interview to mark his 83rd birthday, the veteran president said he had no intention of stepping down.
However civil discontent is rising over the economic crisis, with chronic unemployment and inflation running at more than 1,700% - the highest in the world.
Once the breadbasket of southern Africa, Zimbabwe has had to rely on food imports as agricultural production collapsed after the seizure of white-owned farms was speeded up seven years ago.
Another controversial move, the clearance of slum areas in 2005, left an estimated 700,000 people homeless and affected many more.
A UN report called it inhumane and said it had caused immense suffering among the most vulnerable sections of society.