Police fired rubber bullets to disperse the workers.
The workers have been striking and protesting government privatization plans involving their jobs.
The following is from The Independent (South Africa).
Mandela warns of destructive divisiveness
Former president Nelson Mandela on Tuesday warned against "destructive divisiveness" in the country.
"Remember the horror from which we came. Never forget the greatness of a nation that has overcome its division. Let us never descend into destructive divisiveness."
Mandela was speaking shortly after receiving the Freedom of the City from Tshwane mayor Gwen Ramokgopa at a private ceremony at the Nelson Mandela Foundation in Johannesburg.
Referring to himself as "an old man", Mandela said he had stopped receiving awards but that this award was an exception that he accepted "warmly and with humility".
"We accept this honour from the administrative capital of our nation."
The event was broadcast live to Pretoria's Church Square and the City Hall.
During the celebrations, about 100 striking members of the SA Municipal Worker's Union interrupted proceedings. They sang derogatory songs and called on Mandela to hear their call for better working conditions.
"We want Mandela to hear our call," said union member Thomas Paledi.
Union members have been on strike since last Monday, demanding that the city halt its restructuring process, employ all its workers directly and stop investigations into outsourcing the bus service.
The group also protested outside Pretoria's City Hall shortly around 1pm.
"About 150 striking workers went into the grounds of the hall to disrupt proceedings there. Tshwane Metro police as well as the SA Police Services had to shoot at them with rubber bullets after which they immediately dispersed," said metro police spokesperson Superintendent Alta Fourie.
Fourie said one woman was injured in the incident.
"But she ran away with the protesters," said Fourie, who added that calm was then restored.
The protesters, however, did not dampen the mood of those who came to witness the bestowal of the award on Mandela on a screen set up in Church Square.
A vendor at the square said that the event had increased his sales.
Those who gathered at the square to witness the event via satellite included Alida Boshoff who said the country had come a long way since reconciliation.
"We've come a long way in reconciliation. This award is important and I hope it unites the different people of Pretoria," said Boshoff.
Fourie said roads including Paul Kruger Street leading to the Square would remain closed until around 6pm.
"The celebrations are taking longer than what we anticipated. Roads leading to the Square will be closed until around 6pm but I don't think it will affect afternoon traffic," said Fourie. - Sapa