Violence broke out Tuesday between supporters of South Africa's opposition Democratic Alliance (DA) party, and members of Cosatu, the country's main trade union federation.
Police fired tear gas after clashes outside Cosatu headquarters in central Johannesburg.
Cosatu supporters, in red T-shirts, threw rocks and chunks of cement at DA supporters, wearing blue shirts, the South African Press Association reported.
The DA had organized a march in support of a wage subsidy for young workers, which the party believes will create jobs for more than 400,000 unemployed youths. But Cosatu is opposed to the "Youth Wage Subsidy," arguing that it will mean lower wages and could see older workers laid off.
The DA's march to Cosatu headquarters, to hand over a memorandum in support of the subsidy, had to be called off because of the violence.
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The DA received about 16 percent of the national vote in 2009 elections, and has been pushing to broaden its support base and take on the African National Congress.
The ANC, which has ruled South Africa since the first free elections in 1994, is in a political alliance with Cosatu.
According to SAPA, at least two people were injured in the clashes. A South African journalist reported on Twitter that he was hit by a rock.
SAPA said: "Paramedics could not make their way through the crowd so the injured were taken out on the DA's truck. Cosatu supporters chased the truck down two blocks before police managed to contain the situation."
DA leader Helen Zille, whose speech during the march was interrupted by Cosatu speakers, told the BBC that the police had withdrawn, "leaving our marchers at the mercy of Cosatu."
Lindiwe Mazibuko, the DA's parliamentary leader, accused Cosatu of failing to show leadership.
“I can’t explain how disappointing it is to see that Cosatu showed no leadership in encouraging its members to be peaceful in their opposition to our march. It was absolutely shocking,” she said, according to Eyewitness News.
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