South Africa has urged Zimbabwe to respect the rule of law and rights of citizens, including opposition leaders.
Its call comes after opposition leader Morgan Tsvangirai and others appeared with injuries in a Harare court, after two days in police detention.
US Secretary of State Condoleezza Rice has demanded the immediate release of Mr Tsvangirai and his supporters.
They were arrested when police broke up a rally on Sunday, called to pray over the political and economic crisis.
South Africa, seen as Zimbabwe's most important neighbour, broke its usual silence on the government in Harare to state its concern.
"South Africa urges the Zimbabwean government to ensure that the rule of law including respect for rights of all Zimbabweans and opposition leaders is respected," Deputy Foreign Minister Aziz Pahad said.
As the Movement for Democratic Change (MDC) leader and the other activists were led away from the Harare court to a hospital for treatment, Mr Tsvangirai condemned their treatment by police.
"It's a horrible assault. A sadistic attack on defenceless people," he said.
Speaking to the BBC from the hospital, MP Tendai Biti, secretary-general of Mr Tsvangirai's faction of the MDC, 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 and so forth.
"It was the most vicious assault I have ever witnessed," he said.
Mr Biti said 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 has got a cracked skull and his face is puffed up and he has bruises all over. He must have passed out at least three times."
Twelve of the 50 activists detained have been allowed to stay in hospital overnight for treatment, among them Mr Tsvangirai.
The others were taken back under armed guard late on Tuesday to the court, where they were released into the charge of their lawyers until Wednesday morning, when they will make a further court appearance.
A lawyer for the group, Beatrice Mtetwa, said they would be charged with incitement to violence, the Associated Press news agency reported.
One person was killed when riot police broke up the rally.
The action against Sunday's meeting in Harare - which had been banned - has been strongly condemned by United Nations 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
UN High Commissioner for Human Rights Louise Arbour urged Zimbabwe to conduct an "immediate, impartial and comprehensive investigation" into what happened.
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.
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 the police's action was justified as they were attacked 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.