South Africa's trade union umbrella group, Cosatu, has warned the country is drifting towards dictatorship
In another sign of discontent, a mine union leader backed ex-Deputy President Jacob Zuma as future president, saying adultery is not wrong.
Mr Zuma was recently cleared of rape, but admitted extra-marital sex.
The question over who will succeed President Thabo Mbeki when he stands down in 2009 has divided South Africans and the governing ANC party.
Briefing reporters on the outcome of a Cosatu executive committee meeting, Cosatu general secretary Zwelinzima Vavi said dictatorship never announced its arrival.
"The main concern of the committee centres on signs that we may be drifting towards dictatorship," he said.
"This appears in the use of state institutions... in narrow factional fights," he said.
He said South Africa could be heading down the road Zimbabwe had taken.
The unions have been traditional allies of the government, but splits in the alliance have been exacerbated by Mr Zuma's sacking.
Some feel Mr Zuma would do more to alleviate poverty than Mr Mbeki, who is seen by some unionists as too close to business.
Mr Vavi described the controversy around Mr Zuma - who enjoys strong support from the union movement - as "a symptom, not a cause" of the ANC's "worst crisis in years".
He referred also to the "intimidation" of journalists by senior party officials and the "stifling of debate and closing of democratic space".
Mr Vavi said the governing ANC was currently being run by cabinet ministers and business people.
"We are fighting for the ANC to retain its pro-worker bias," he said.
Cosatu has 1.8m paid-up members.
Earlier, National Union of Mineworkers President Senzeni Zokwana, who represents one of South Africa's most powerful unions, rejected President Mbeki's suggestion that the country's next leader should be a woman.
"We are not Christians," Mr Zokwana told the union's 12th national congress being held in Midrand, near Johannesburg.
"We don't listen to the Ten Commandments and we don't have to listen when Christians tell us adultery is wrong.
"We also don't need Christians to tell us who our leaders should be."
"We can't have people telling us that our president must be a woman," Mr Zokwana said.
Mr Mbeki recently suggested South Africa's next leader should be a woman: the clearest indication he has given so far that he does not favour Mr Zuma as a successor.
Mr Zuma was seen as a presidential heir apparent before corruption charges and later a rape charge were brought against him last year.
The corruption charges, which he denies, are due to be heard in July.
After being cleared of rape, Mr Zuma was permitted to resume his ANC leadership duties, from which he was suspended when the rape charge was laid.
Mineworkers slam 'adultery opinion' on Zuma