The leader of Zimbabwe's opposition Movement for Democratic Change (MDC) is in a bad state after being beaten by police in custody, says his lawyer.
Zimbabwe officials said the opposition had attacked police
Morgan Tsvangirai's head was bandaged and his face swollen, said Innocent Chagonda, who saw him from a distance.
Mr Tsvangirai was arrested on Sunday along with several colleagues after riot police broke up a public meeting.
A high court judge on Monday said Mr Tsvangirai must be allowed access to lawyers and treated immediately.
The judge also said that Mr Tsvangirai should be brought before his court on Tuesday by 0800 on Tuesday or released from police custody.
Riot police have been out in force in the capital, Harare.
As reports spread of the beatings, demonstrations erupted in Gweru, about 200km south of the capital, Harare, and hundreds were arrested in the eastern town of Mutare, say the BBC's Brian Hungwe.
Lawyers are seeking access to the detained men and it is unclear if any have been charged. The Save Zimbabwe Campaign, which organised the meeting says they have also been tortured.
One prominent government critic, Lovemore Madhuku, suffered a broken arm and had to be rushed to hospital after passing out, the civic grouping said.
Save Zimbabwe Campaign lawyers report that Mr Tsvangirai fainted three times in custody.
The country's information minister said the police themselves had been beaten.
"The opposition has been involved in violence, caught by police with weapons of destruction and destroying cars and stores and beating up people," Sikhanyiso Ndlovu told the BBC's Network Africa programme.
"They've been beating up police you know. That is what government cannot tolerate."
Mr Tsvangirai has previously been charged with treason
One activist was shot dead at the rally, described as a prayer meeting, in the capital, Harare. Crowds gathered for his funeral in Glenview township singing protest songs.
The government said the rally breached a ban on political gatherings.
The US condemned the crackdown and called for the release of those in custody.
"The United States government condemns the brutal and unwarranted actions of the government of Zimbabwe [on] March 11 in attacking its citizens peacefully gathered to exercise their legitimate democratic rights," spokesman Sean McCormack said.
"We hold President Robert Mugabe and the government of Zimbabwe accountable for the government's actions today, and for the safety and well-being of those in custody."
However, there has been no word so far from any of Zimbabwe's neighbours.
MDC spokesman in London Hebson Makuvise said Mr Tsvangirai was arrested at around 1300 local time (1100 GMT) on Sunday.
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 dropped
2002: Lost elections to Mugabe, charged with treason - later dropped
2003: Charged with treason - later dropped
He told the BBC that the MDC leader was beaten unconscious and could hardly walk or open his eyes.
"He was severely assaulted. He can hardly walk and can hardly open his eyes. We don't know the extent of his injuries," he told BBC Radio 4's Today programme.
Mr Tsvangirai has been arrested several times since leading the opposition to Mr Mugabe.
A group of unknown men once broke into his office, hit him on the head with an iron bar and tried to throw him out of the window.
He has three times been charged with treason but these have all subsequently been dropped.
BBC Southern Africa correspondent Peter Biles says the MDC, which split into two rival factions more than a year ago, came together at the weekend in a rare show of unity aimed at trying to resolve the political and economic crisis in the country.
They were joined by civic and religious organisations in the Save Zimbabwe Campaign.
Riot police used tear gas and water cannons as they fought battles with activists gathered for the rally, opposition officials said.
A police spokesman said warning shots were fired after the group defied orders to disperse.
MDC spokesman Eliphas Mukonoweshuro said the rally was not political and therefore not subject to the provisions of the public order act.
A three-month ban on political meetings was imposed after violence at an opposition rally last month.
Robert Mugabe, who has led Zimbabwe since independence in 1980, retains an iron grip on power.
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.