A march is being held to protest against cuts and privatisation by a Conservative-run local authority.
Len McCluskey, leader of Unite, will join the demonstration on Saturday in the London borough of Bromley, saying the union and local residents opposed cuts and the sell-off of libraries, schools, services for the elderly and the disabled.
He said: "Bromley council is failing the people of Bromley. I am proud that Unite is opposing the cuts.
"In Bromley alone, we have members who have taken over 30 days of strike action in defence of the services they provide.
"Just this week Unite members working in the libraries of Bromley stood up and fought back because they oppose plans to sack staff and cut library services."