London bus workers have been protesting about a lack of toilets for drivers.
London has about 23,000 bus drivers
Members of the Unite union claim there are too few facilities on their routes and at their workplaces.
The protest started at Victoria Coach Station in central London and was due to spread to key London transport and local government offices.
Transport for London (TfL) said it recognised the importance of adequate toilet facilities and was working hard to provide them for bus staff.
The workers have threatened a full strike ballot if TfL does not speed up action to provide facilities.
Unite organiser Peter Kavanagh said: "We know the issue can be good for a laugh but the lack of facilities for these essential London workers is no joke."
Mr Kavanagh said that earlier this week a driver who went without a break for three hours had to use facilities at a local shopping centre and then was reported for delaying the service.
He went on: "Some drivers have been so desperate for a toilet break they have had to relieve themselves in public places and been arrested."
London's mayor Ken Livingstone has backed the union's calls for improved toilet facilities.
He said that he had asked TfL "to do everything in their power to solve this problem as rapidly as possible".
23,000 bus drivers
A spokesman for TfL said: "We recognise that the provision of adequate toilet facilities for bus drivers in London is vital.
"We have made a lot of progress in recent years, and are now in a position where more than 95% of routes have toilet facilities at one end of the route at least.
"Making sure that the remaining routes, about 25 in total, have proper facilities is a top priority for this year, and we have a budget of £1m dedicated to the provision of toilet facilities on these routes."
According to TfL, there are about 23,000 bus drivers in London.