The South African Municipal Workers' Union (Samwu) will march in Johannesburg on Friday demanding an 18% salary increase, the union said.

Provincial chairperson Koena Ramotlou said union members would gather at Peter Rose Park in the city at 9am and then march to the SA Local Government Association (Salga) offices. The march would then proceed to the Premier's Office in central Johannesburg.

"We expect 60% of our members to participate in the march," Ramotlou said.

Samwu took the streets on Monday demanding an 18% pay rise while employers, Salga, were offering 6.08%.

Earlier this week, the union said that municipal workers would bring the Johannesburg CBD to a standstill during Friday's protest march.

"We expect the town to be standing still... we will fill Simmonds [street] from the top," Gauteng provincial secretary Ntsikelelo Klaas told journalists on Wednesday.

On Thursday Samwu spokesperson Tahir Sema said that workers would march for their wage demands and also in protest against the Municipal Systems Amendment Bill.

The legislation is being called the cadres bill because it bars party office bearers from being employed in senior positions in municipalities.

Samwu said it was still deciding whether to challenge the bill in the Constitutional Court.

The union called off a threatened strike about the bill a week before the local government elections on May 18 after last-minute talks with the ANC, led by President Jacob Zuma.

Johannesburg metro police warned that Friday's march would affect traffic flow in the city.

Motorists were advised to avoid streets around the Metro Centre in Braamfontein.

Johannesburg metro police spokesperson Chief Superintendent Wayne Minnaar said motorists should avoid Jorissen street, Rissik street north of Jorissen and Empire road.