Hebson Makuvise told the BBC Morgan Tsvangirai was beaten unconscious and could hardly walk or open his eyes.
Lawyers are trying to get access to Mr Tsvangirai, who was arrested on Sunday along with five colleagues after riot police broke up a public meeting.
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.
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| Riot police have blocked all access to the Highfield township stadium [AP] |
Officials had earlier said that Arthur Mutambara, who leads another faction of the MDC, and Lovemore Madhuku, of the pressure group National Constitutional Assembly, were also detained.
Riot police moved in force early on Sunday to head off the prayer rally, which police had said would violate a ban on political protests imposed after opposition supporters clashed with police in Highfield last month.
The organiser, opposition group Save Zimbabwe Coalition, had argued that the ban should not apply to a prayer vigil.
"Highfield has been turned into a war zone," it said.
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.
(photo: Desmond Kwande/Getty Images)
Economic crisis
Bvudzijena said he had no immediate information on the arrests.
Officials have increased curbs on opposition movements after violence broke out last month when riot police broke up an MDC rally despite a court order directing that it should be allowed.
State media said officials feared the rally was intended to launch street protests against Mugabe's government.
Mugabe, 83, and in power since independence in 1980, dismisses the MDC as a puppet of Zimbabwe's former colonial master Britain.
Agencies
