Zimbabwean opposition leader Morgan Tsvangirai has pledged to continue his "struggle" after he and other activists were beaten while in police custody.
Mr Tsvangirai had a swollen face when he appeared in court
Mr Tsvangirai and 12 of his supporters have been treated in hospital after what he called a "sadistic attack".
The activists were arrested on Sunday when police broke up a rally in Harare which the authorities had banned.
They have all now been released from police custody, their lawyer said, but are due back in court later.
Home Affairs Minister Kembo Mohadi told the state-run Herald newspaper that the law would be enforced "to its fullest".
Mr Tsvangirai - who heads the Movement for Democratic Change (MDC) - was among 50 opposition activists who appeared in a Harare court on Tuesday, following two days in police detention.
As he and others were led away from the Harare court, he condemned their treatment by police.
"The police assaulted defenceless civilians but the struggle continues," Mr Tsvangirai said.
Speaking to the BBC from a hospital, MDC official Tendai Biti described how he and his colleagues were beaten up.
"I can hardly walk. They were hitting me on the buttocks and on my legs, the fibula. I was assaulted while I was lying prostrate.
"There were others who were assaulted while standing and in the process tried to defend themselves, which is why we've got so many people with fractures of the hand."
Mr Biti said it was "the most vicious assault" he had ever witnessed, adding that Mr Tsvangirai was in "a very bad condition".
"I saw him being assaulted. There was a time when for 15 minutes they were assaulting him with their baton sticks non-stop. He must have passed out at least three times," he said.
Twelve of the 50 suspects were allowed to stay in hospital overnight for treatment, among them Mr Tsvangirai.
The others were released into the charge of their lawyers until Wednesday morning, when they are due to appear in court again.
The men are expected to be charged with incitement to violence. They tried to hold what they described as a prayer meeting in Harare on Sunday.
In a rare criticism of its neighbour, South Africa urged Zimbabwe's President Robert Mugabe to respect the rights of citizens, including opposition leaders.
The action against Sunday's demonstration was strongly condemned by the UN Secretary General, Ban Ki-moon, as well as the US 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
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.
UN High Commissioner for Human Rights Louise Arbour urged Zimbabwe to conduct an "immediate, impartial and comprehensive investigation" into what happened.
However, few of Zimbabwe's neighbours have condemned its policies.
President Levy Mwanawasa of neighbouring Zambia voiced his concern about the situation, but said problems in Zimbabwe were for its own people to solve.
Zimbabwe's Information Minister, Sikhanyiso Ndlovu, told the BBC that the police officers' action was justified as they were under attack by opposition activists.
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.