Around 40 staff from McDonald's branches in Cambridge and Crayford, south-east London, began a 24-hour action at midnight, a first-ever UK strike to hit the multi-billion pound firm.
The group rallied outside the Houses of Parliament to protest against zero-hour contracts and lack of union recognition. They were supported by union workers and members of parliament, including the shadow chancellor, John McDonnell.
The Bakers, Food and Allied Workers Union said staff wanted a wage of at least £10 an hour and more secure jobs. McDonald's said the action related to internal grievance procedures, not pay.
Ian Hodson, the union's president, said, 'We fully support the historic decision by these brave workers to stand up and fight back against McDonald's - a company that has let them down one too many times.
'For far too long, workers in fast food restaurants such as McDonald's have had to deal with poor working conditions, drastic cuts to employee hours, and even bullying in the workplace - viewed by many as a punishment for joining a union.'
The worldwide chain had £19bn sales last year and employs 375,00 staff across the globe, including 85,000 in Britain.