Zimbabwe's opposition leader Morgan Tsvangirai has been taken to hospital after appearing in court with injuries following two days in police detention.
He was taken from Harare magistrates court with several other activists who were detained at an opposition rally on Sunday. Many of them were bandaged.
Mr Tsvangirai accused the police of "a sadistic attack on defenceless people".
One person was shot dead as riot police broke up the meeting, called to pray for the political and economic crisis.
In a rare public comment on its neighbour's government, South Africa has called on Zimbabwe to respect the rule of law.
"South Africa urges the Zimbabwean government to ensure that the rule of law, including respect for rights of all Zimbabweans and leaders of various political parties, is respected," said deputy foreign minister Aziz Pahad.
BBC reporter Brian Hungwe, who was at the courthouse, said the Movement for Democratic Change (MDC) leader appeared with 10 of about 50 detainees who were brought to the court.
He says Mr Tsvangirai was badly swollen, his right eye was shut, he had deep cuts and several stitches and could barely walk.
The other detainees had similar injuries, our reporter says.
Mr Tsvangirai stood next to Arthur Mutambara, the leader of a rival MDC faction, while many of the activists sang and chanted, AFP news agency reported.
A lawyer for the group, Beatrice Mtetwa, said they would be charged with incitement to violence, the Associated Press news agency reported.
The detainees were taken to hospital after prosecutor Florence Ziyambi said they would be allowed to receive medical attention.
The action against Sunday's meeting in Harare, which the police say was banned, has been strongly condemned by United Nations Secretary General Ban Ki-moon, and by the United States and European Union.
TSVANGIRAI'S LEGAL TROUBLES
2003: Charged with treason - later dropped
2002: Lost election to Mugabe, charged with treason - later dropped
2000: Charged with treason - later dropped
2000: MDC won 57 parliamentary seats
1999: Helped form MDC
Mr Ban's spokeswoman said the arrests "violate the basic democratic right of citizens to engage in peaceful assembly".
The UN High Commissioner for Human Rights, Louise Arbour, urged Zimbabwe to conduct an "immediate, impartial and comprehensive investigation" into what happened.
US Secretary of State Condoleezza Rice called for Mr Tsvangirai's immediate release.
"The world community again has been shown that the regime of Robert Mugabe is ruthless and repressive and creates only suffering for the people of Zimbabwe," she said.
Zimbabwe's Information Minister Sikhanyiso Ndlovu said the police's action was justified as they were attacked by opposition activists.
"The opposition has been involved in violence, caught by police with weapons of destruction and destroying cars and stores and beating up people," he told the BBC's Network Africa programme.
"They've been beating up police you know. That is what government cannot tolerate."
The government said the rally breached a ban on political gatherings imposed after violence at a demonstration last month.
Civil discontent in Zimbabwe is rising over the country's economic crisis, with chronic unemployment and inflation running at more than 1,700% - the highest in the world.