Zimbabwe's High Court has ordered that arrested opposition leader Morgan Tsvangirai either be delivered to court by lunchtime or released.
Morgan Tsvangirai's whereabouts are unclear
Supporters say Mr Tsvangirai and others arrested at a public meeting on Sunday have been badly beaten.
Lawyers say police have already defied a court order granting access to the activists, and are waiting to see if they are brought to court.
There has also been no word on another opposition leader, Arthur Mutambara.
Mr Tsvangirai, who leads the Movement for Democratic Change (MDC), was detained along with other opposition members as riot police broke up Sunday's rally in the capital Harare.
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
One activist was shot dead.
A high court judge ordered that those arrested should be taken for immediate medical treatment.
Mr Tsvangirai's lawyer, Selby Hwacha, told the BBC his staff had visited a number of police stations in Harare on Monday but had been turned away.
He said he believed Mr Tsvangirai had been badly beaten in custody and there was no indication he had been granted medical treatment.
"It's quite clear now that he was beaten up, and reports reaching us are that he was beaten up very badly. I received information which stated his head was cracked," he said.
United Nations Secretary General Ban Ki-moon condemned the arrests, which his spokeswoman said "violate the basic democratic right of citizens to engage in peaceful assembly."
The US condemned the "brutal and unwarranted" crackdown and called for the activists to be released.
"We hold President Robert Mugabe and the government of Zimbabwe accountable for the safety and well-being of those in custody," White House spokesman Sean McCormack said.
Zimbabwe officials said the opposition had attacked police
However, Zimbabwe's Information Minister Sikhanyiso Ndlovu told the BBC's Network Africa programme that the police had been 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.
"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.
The BBC's Africa correspondent, Peter Biles, says the treatment the detainees are alleged to have received at the hands of the Zimbabwean police appears to represent an even more determined effort to crack down on any opposition to Robert Mugabe's government.
Mr Mugabe, 83, who has led Zimbabwe since independence in 1980, retains a firm grip on power and recently said he had no intention of stepping down.
However civil discontent is rising over the country's economic crisis, with chronic unemployment and inflation running at more than 1,700% - the highest in the world.