Correctional Services said that “matters are under control” at Johannesburg’s Sun City Prison on Wed.
The spokesman for Zimbabwe's Movement for Democratic Change, Nelson Chamisa, was assaulted by security forces as he tried to leave the country, an opposition official said, accusing the government of continuing to target dissident activists.
President Robert Mugabe's government is under increasing international criticism for its treatment of the country's opposition. Activists said the government has been disrupting their gatherings and beating and detaining their leaders.
Three opposition activists who were allegedly assaulted when police broke up a protest meeting last Saturday were rearrested at Harare International Airport at the weekend.
On Sunday Chamisa, an aide to MDC leader Morgan Tsvangirai, was assaulted at the airport as he was trying to leave for a meeting abroad, said the party's secretary-general, Tendai Biti.
"He was beaten on the head with iron bars. There was blood all over his face. He is in a critical condition at a private hospital in Harare."
Tsvangirai said the crisis in Zimbabwe had reached a decisive moment.
"Things are bad," he said, "but I think that this crisis has reached a tipping point, and we could see the beginning of the end of this dictatorship."
Mugabe, meanwhile, accused the opposition of being terrorists supported by Britain and the West.
In Saturday's arrests, activists Grace Kwinje and Sekai Holland were prevented from leaving to seek medical care in South Africa.
Holland, 64, was completely immobilised on her left side and suffered multiple fractures, including a broken arm and leg and three broken ribs, reports said.
The women's passports were taken and they were told they needed clearance from Zimbabwe's Health Department.
Arthur Mutambara, also an activist, was being held by police on charges of inciting public violence, said his lawyer, Harrison Nkomo.
Germany, which holds the EU presidency, said it was "outraged and deeply concerned" by news of the beatings and arrests, and called on the government to release activists now in police custody.
The latest violence has drawn new attention to a deteriorating situation in Zimbabwe, where the increasingly autocratic Mugabe is blamed by opponents for repression, corruption, acute food shortages and inflation of 1600percent - the highest in the world. - Sapa-AP