Zimbabwean opposition leader Morgan Tsvangirai is being treated in an intensive care unit as doctors examine wounds he received in police custody.
He has been scanned amid fears he may have a fractured skull, and received two pints of blood overnight, his spokesman told the BBC News website.
He and dozens of other activists were arrested at a rally on Sunday.
He was due to appear in court on Wednesday morning, but was too ill to attend. The hearing was abandoned.
"I was at court this morning and we were told to go away. There seems to be confusion over the state of the charge," said his spokesman William Bango.
Mr Tsvangirai and 49 others appeared in court in the capital, Harare, on Tuesday, but have all been released from police custody over the past 24 hours, their lawyers said.
A number remained in hospital on Wednesday.
On leaving court on Tuesday, Mr Tsvangirai condemned their treatment.
"The police assaulted defenceless civilians but the struggle continues," said Mr Tsvangirai, who heads the Movement for Democratic Change (MDC).
Speaking to the BBC, MDC colleague Tendai Biti described how Mr Tsvangirai was beaten.
"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.
"I can hardly walk," he said from a hospital.
"They were hitting me on the buttocks and on my legs, the fibula. I was assaulted while I was lying prostrate."
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.
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.