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...."