About 40 staff staged a walkout on Monday at restaurants in Cambridge and Crayford, south-east London, over low wages and zero hours contracts, with some of the global chain's workers on wages as low as £4.75 per hour.
The strike was engaged in with other fast food workers across the country. Leader of the Labour party, Jeremy Corbyn is backing the workers.
Monday's action coincides with strikes in the United States and Belgium. Two managers in the Cambridge branch of the restaurant were suspended this year for bullying and harassment, the union said.
McDonald's is one of the biggest users of zero hour contracts in Britain.
"We fully support the historic decision by these fearless workers to stand up and fight back against McDonald's - a company that has let them down one too many times", Ian Hodson, national president of the Bakers, Food and Allied Workers Union (BFAWU) told the BBC.
Jeremy Corbyn, the leader of Britain's main opposition Labour Party, said he supported "the courageous McDonald's workers, who are making history today".
McDonald's highlighted that those on strike made up less than 0.01 percent of its workforce and that 86 percent of its workers had opted for flexible contracts.
A spokesman told Reuters that the strikers represented less than half of the 33 union members who were balloted, and the actual reason for the strike related to "internal grievance procedures", not to pay or contracts.
"Their demands - an end to zero-hours contracts by the end of the year, union recognition and a £10 per hour minimum wage - are just, and should be met", Corbyn said.
"There are 800,000 people, approximately, in Britain on zero hours contracts".
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