"The secretary-general urges the government of Zimbabwe to release the detainees and to guarantee their safety"
Spokesperson for Ban Ki-moon,
the UN chief
Tsvangirai and other opposition officials were arrested on Sunday after police blocked their motor convoy from driving to a stadium where a prayer vigil was due to be held.
Haru Mutasa, Al Jazeera's correspondent in Harare, spoke to Alec Muchadahama, a lawyer for Tsvangirai.
"He is swollen all over the body and he has collapsed three times," he said.
Ban Ki-moon, the UN secretary-general, condemned the reported police brutality as a violation of the right to peaceful assembly, according to his spokesperson, Michele Montas.
"The secretary-general urges the government of Zimbabwe to release the detainees and to guarantee their safety," she said.
The EU criticised the suppression of the demonstration and condemned "the ongoing violent suppression of the freedom of opinion and of assembly, as well as of other fundamental rights".
The US State Department described the attacks as "an indication of the repressive nature of the Mugabe dictatorship", according to spokesman Tom Casey.
South Africa's government, which prefers quiet diplomacy to sway Robert Mugabe, the president, has remained silent but the country's trade unionists and opposition parties have voiced outrage over Monday's incident.
Tsvangirai is expected to be charged or released by Tuesday.
His wife, Susan, who was allowed to see him in a suburban jail on Monday reported the wounds, some of which were sutured.
Lovemore Madhuku, another opposition leader, is reported to be in serious condition in a Harare hospital.
A Harare freelance photographer, Tsvangirayi Mukwahzi, and a freelance television producer, Tendai Nusiyu, were among the journalists arrested at the scene.
No comment was immediately available from police on the latest arrests.
Innocent Chagonda, another lawyer who visited the Harare police station, said of Tsvangirai: "He was in bad shape, he was swollen very badly. He was bandaged on the head. You couldn't distinguish between the head and the face and he could not see properly.
"I managed to see him from about 10 metres inside the police holding-fence at Borrowdale police station. They were being paraded," he added.
One government minister told Al Jazeera that opposition supporters had got what they deserved.
"Tsvangirai will never have a good word to say about his country," Skhanyiso Ndlovu, the minister of information, said. "He is a black stooge, he goes around asking for sanctions on his own people.
"There is no future for them [MDC]. People are fed up with opposition groups who pocket money from donors."
Witnesses said police also clashed with opposition supporters around the venue in the Harare township of Highfield.
Police said one protester was killed when 200 "thugs" attacked 20 policemen.
But Al Jazeera's correspondent said she did not see any MDC members carrying weapons while on the way to the prayer vigil, adding that the only people she saw carrying arms were the police.
A statement from the MDC said a party official was "shot and killed in cold blood" by police who were now "using live ammunition on innocent people".