The police formed a human chain by linking hands in front of Cosatu supporters to stop them from getting to DA marchers in Johannesburg on Tuesday.
Democratic Alliance protesters taking part in the march earlier took off their blue shirts in fear of being targeted by Congress of SA Trade Unions supporters.
While marching down Jorissen Street in Braamfontein, they were met head-on by Cosatu supporters wearing red T-shirts.
Cosatu supporters intimidated the DA crowd and stopped them from proceeding to Cosatu House.
At least two people were injured when rocks were thrown. Paramedics could not make their way through the crowd so the injured were taken out on the DA's truck.
Parliamentary leader Lindiwe Mazibuko told the crowd they were on the same side, fighting for economic freedom.
“I felt sad that South Africans, who fought for tolerance and freedom, couldn't exercise those rights today,” she said.
Rocks and broken branches littered the road.
Earlier there was a loud bang, believed to be the police firing teargas at Cosatu supporters.
In a statement, the DA's Kate Lorimer said there were not enough police and metro police officers present.
“The legal DA march has been met by an illegal gathering of violent Numsa and Cosatu members who are throwing bricks and stones at peaceful DA marchers,” Lorimer said.
“It seems though as if the cops were unprepared and it was only after the stone throwing that the SAPS finally sent in a police vehicle to block the Cosatu attack.”
Several hundred DA supporters earlier started their march at Beyers Naude Square in the city centre to protest against the union federation's opposition to the youth wage subsidy.
DA leader Helen Zille, youth leader Makashule Gana, Mazibuko and national spokesman Mmusi Maimane led the protest. The planned route was along President, Rissik, Jeppe, Sauer, Burger, Jorissen, Melle, and Simmonds streets.
Meanwhile, Cosatu supporters in red T-shirts waited outside the federation's offices.
“We are here to welcome the DA when they arrive,” Cosatu spokesman Patrick Craven said.
Earlier, police said the protesters could not end their march outside Cosatu House in Braamfontein. Instead it was agreed they would finish at least 50m away, on the lawns of the Joburg (formerly the Civic) Theatre.
The DA believes the youth wage subsidy will create at least 423,000 jobs for young, unemployed South Africans.- Sapa