MPs and peers faced picket lines on Wednesday as Houses of Parliament cleaners staged their first strike.
The cleaners are calling for a wage of £6.70 an hour
Up to 170 members of the Transport and General Workers' Union (TGWU) walked out in a row over pay and holidays.
The cleaners, who are employed by two agencies, earn £5 an hour, have no sick pay or company pension scheme and receive 12 days' holiday a year.
MPs and Lords were warned they would have to clean their own toilets and offices unless the dispute was settled.
About 20 Labour MPs joined the demonstration and pledged to support the workers' demand for a pay rise.
Dozens of cleaners stood outside the main entrance to the Commons brandishing mops and banners and chanting "low pay, no way" and "cleaners united will never be defeated".
Jack Dromey, deputy general secretary of the Transport and General Workers' Union (TGWU), said on the picket line: "What is going on inside Parliament is a national scandal.
"Parliament is paying poverty wages, which is shameful."
The dispute was raised during Wednesday's Prime Minister's Questions, when Tony Blair said the pay of Commons cleaners was a "commercial matter" and a matter for the house authorities.
The cleaners are calling for a "London living wage" of £6.70 an hour and say they want sick pay and better holidays.
One of the cleaners, Tesfaalen Gebru, 43, said he had two cleaning jobs at the Commons to earn a decent wage.
He cleaned kitchens during the night and then cleared rubbish during the day, working a 64 hour week.
"We are working in the most privileged house in the world. We want to do a great job, but the money does not reflect our work.
"Many people here do two jobs most of the time, and I really think we deserve a decent pay rise."
John Cruddas, Labour MP for Dagenham in east London, who joined cleaners on the picket line, said he believed most MPs supported their case.
"This is a classic symbol of how public service workers keep this place going," he said.