Workers who clean offices in London's Canary Wharf complex are protesting over pay and holiday entitlements.
Up to 100 cleaners stood outside finance firm Morgan Stanley in Docklands demanding a £1-an-hour pay rise, taking their wages to £6.70.
They are also calling for holidays to be increased from 12 days a year to 20.
Up to 300 cleaners, employed by contractors to work at the Morgan Stanley offices, belong to the union.
The company declined to comment.
A letter has also been sent to the chief executive officer of Morgan Stanley asking him to ensure the cleaners are paid a "living wage".
Jack Dromey of the Transport and General Workers' Union (TGWU) said it was wrong that people work night and day to make ends meet.