Cosatu and the DA have called for a police investigation into the alleged arson attack at Luthuli House, the ANC's headquarters.
Cosatu expressed shock at Tuesday's fire in the lobby of the ANC's headquarters in Johannesburg.
"[We are] shocked and outraged at the planting of a fire-bomb at Luthuli House," Cosatu spokesperson Patrick Craven said in a statement on Wednesday.
"We congratulate the security staff whose swift action to quench the flames prevented what could have been a huge disaster."
Cosatu urged police to investigate a case of attempted murder. "Had the fire spread throughout the building, many lives could have been lost. This is not just an attack on the ANC, but on our constitutional democracy," he said.
Earlier, the Democratic Alliance condemned the suspected arson attack. "Such incidents undermine the spirit of our democracy and the freedom of political parties to campaign without fear of intimidation," spokesperson Mmusi Maimane said in a statement.
"The Democratic Alliance hopes the perpetrators of this alleged crime will be caught and prosecuted in terms of the law," he said.
The ruling party said the fire at its Johannesburg headquarters was a malicious act. A bottle believed to be filled with chemicals was placed under a couch. The couch caught fire as the bottle exploded, said ANC spokesperson Jackson Mthembu on Wednesday.
"In our books that is grounds to be confident that this was arson," he said.
Mthembu said the chemicals that filled the bottle left fumes still lingering in the building. A bomb unit, police and other experts all confirmed that the attack was indeed an arson attempt.
The ANC said it viewed such actions in a serious light and expressed disappointment at the act. "Such actions do not have a place in a democratic nation. We have avenues to address issues and this [attempted arson] is not one of them ... This was a criminal act.
"South Africa does not need such incidents. An attack at the offices of the governing party or any political office under a democracy is something we do not need," said Mthembu.
Maimane echoed the ANC's sentiments. "In a country where constitutional freedoms have been achieved through great sacrifice, we cannot allow such actions to take us back to a place of fear or intolerance."
The ANC said it was unclear what motives were behind the attack.
Police brigadier Neville Malila confirmed an investigation into an alleged arson attack was underway. – Sapa