The hostility displayed by Cosatu members during a DA march in the Johannesburg city centre on Tuesday was justified, the trade federation said.
"DA policies... will create no new jobs... and [will] lead to the impoverishment and enslavement of workers and [an] economic meltdown for South Africa," spokesman Patrick Craven said in a statement.
"That is why workers are so hostile towards [DA leader] Helen Zille's party and want to express their justified anger, most especially when there are confronted on the street outside their federation's head office," he said.
Earlier on Tuesday, police used teargas and water cannons to disperse Congress of SA Trade Union members who stopped the Democratic Alliance from protesting outside its headquarters.
The DA wanted Cosatu to stop stonewalling a government youth wage subsidy which it believes will create thousands of new jobs for young people.
The DA march, headed by Zille, youth leader Makashule Gana, parliamentary leader Lindiwe Mazibuko and national spokesman Mmusi Maimane, began peacefully.
The police kept the groups of DA and Cosatu members apart for an hour while they traded insults.
When rocks and pieces of cement were thrown into the crowd during Zille's speech and a DA member on the VIP truck was seriously injured, the DA retreated.
Craven congratulated Cosatu members for causing the DA to make an "ignominious retreat".
"Regrettably, there was some confusion amongst the demonstrators as to precisely where the DA's memorandum would be handed over," he said.
"This led to a confrontation... during which there were a few incidents of stone-throwing by individuals, which led to a number of injuries by people on both sides of the confrontation. We wish them all a full recovery."
He said Cosatu condemned the violence, but said most of its members had "conducted themselves "with exemplary discipline and restraint".
"We recognise the DA's right to demonstrate, but insist that Cosatu members have an equal right to counter-demonstrate."
In a counter-memorandum, Cosatu said the majority of South African workers were opposed to all DA policies, as they reflected the interests of its "backers in big business".
"The basis for these differences are rooted in the fact that while Cosatu represents the working class and the poor, the DA speaks for big business, the wealthy and the privileged," it said in its memorandum.
"That is why we can see no benefit in discussing these matters with them [the DA]."
Zille, who addressed marchers in Xhosa and in English, said the DA was marching in solidarity with unemployed South Africans.
"We all want to live lives that contribute to society. We all want to support ourselves and our families. We all want to ensure that our children have more opportunities than we did," she said.
A few blocks away at Cosatu House, Cosatu general secretary Zwelinzima Vavi told a crowd that a youth wage subsidy would use workers' tax money to enrich company bosses and had to be opposed.
"What will happen is when workers get old? Bosses will throw them into the street and replace them with 18-year-olds receiving a subsidy," Vavi said.