They are campaigning for the London Living Wage and the right to sick pay.
One placard in Spanish said, “We’re cleaners, not slaves.”
The protest interrupted the University of London’s open day at Senate House as cleaners and supporters banged drums and chanted.
Frank Dobson MP told the protest, “If management claim they haven’t got the money to pay the London Living Wage, then we have to get it back off the bankers.”
The protest was called off the back of a victorious unofficial strike earlier this month, which forced Balfour Beatty to pay all the wages owed to the cleaners.
Despite this, the company threatened the cleaners with suspension if they protested again.
One cleaner, who asked not to be named, said, “We won within hours last time. We have to strike again.”