Fast-food workers have staged strikes at McDonald's and Burger Kings and demonstrated at other stores in 60 US cities in their latest action in an almost year-long campaign to raise wages in the service sector.

The strikes spread quickly across the US and shut down restaurants in New York, Chicago, Detroit, Milwaukee, St Louis, Raleigh and Seattle, according to organisers.

The fast-food workers were expected to be joined by retail staff from stores owned by Macy's Inc, Sears Holdings Corp and Dollar Tree Inc.

The fast-food workers want to form unions in the virtually union-free sector without employer retaliation and bargain for higher wages.

They are demanding pay of $15 (€11.33) an hour, up from $7.25 (€5.48), which is the current federal minimum wage.

Martin Rafanan, a community organiser in St Louis, said local employees of McDonald's and Wendy's can not make it on the salaries.

"If you're paying $7.35 an hour and employing someone for 20, 25 hours a week, which is the average here, they're bringing home about $10,000 a year. You can't survive on that," Mr Rafanan said. Missouri's minimum wage is $7.35 an hour.

He added: "Unless we can figure out how to make highly profitable companies pay a fair wage to their workers, we're just going to watch them pull all the blood, sweat, tears and money out of our communities."

McDonald's profits totalled $5.47 billion in 2012.

Momentum has been building in recent months, organisers say, as they receive financial and technical support from the Service Employees International Union, community activists, politicians and the clergy.

Last November, around 200 workers walked off their fast-food jobs in New York City. Groups in Chicago, Kansas City, Detroit and other cities followed their lead in April and July.

The $200 billion US fast-food sector, as well as retail sales and food preparation, have been under the spotlight because they have added most of the jobs, in many cases lower-paying and part time, since the recession.

Restaurant chains and trade groups say the protests are unwarranted because fast-food and retail outlets provide Americans with millions of good jobs with competitive pay and ample opportunities to rise through the ranks.

"Our history is full of examples of individuals who worked their first job with McDonald's and went on to successful careers both within and outside of McDonald's," McDonald's said in a statement.

Wendy's and Burger King did not respond to requests for comment.

The restaurant chains have not changed their wage policies as a result of recent strikes.

The National Retail Federation said in a statement the strikes are "further proof that the labour movement (has) abdicated their role in an honest and rational discussion about the American workforce".

In today's Wall Street Journal, the conservative Employment Policies Institute ran a full-page ad with a picture of a robot making pancakes, warning that higher wages would mean "fewer entry-level jobs and more automated alternatives".

"You can either raise prices and lose customers, or (automate) those jobs," said Michael Saltsman, EPI's research director, adding that "the idea that restaurants are rolling in the money is not representative of the situation franchisees face".

The median wage for front-line fast-food workers is $8.94 per hour, according to an analysis of government data by the National Employment Law Project (NELP), an advocacy group for lower-wage workers.

"The workers are responding to total failure on behalf of the federal government to raise the minimum wage to keep up with inflation and the cost of living," said Tsedeye Gebreselassie, a lawyer at the NELP, referring to the strikes.

The walkouts, coming before the US Labor Day holiday on Monday, also took place in the Southern states of Texas, Louisiana, and North Carolina.