Thousands went on strike in cities including New York, Chicago, Los Angeles, Detroit, Milwaukee and Indianapolis, according to local organisers and the Service Employees International Union, which said it was the largest protest ever to hit the fast-food industry.

The protesters called for union representation and an increase in wages to $US15 an hour ($16.80), more than double the federal minimum wage.

''The economy is doing poorly. Everything is expensive. With high taxes, we're not going to be able to pay rent,'' Domino's worker Francisco Zuniga, 34 said at a protest in Hollywood.

Mr Zuniga said he could not support his family on $US8 hourly pay from his pizza-making, order-taking and delivery job at Domino's.

Teenagers used to dominate fast-food jobs. But now many older workers, out of a job because of the stagnant economy, have gravitated towards the industry. A sociology professor at the University of North Carolina, Arne Kalleberg, said the median age for fast-food workers was more than 28; for women it was 32. And low-wage jobs were among the fastest-growing in the country, he said.

''These protests are a cry for help,'' he said. ''It's a microcosm of a larger phenomenon.

''It reflects the growing frustration of these folks who for a long time have seen the gap between what they're earning and the tonnes of money the corporations and the CEOs are making.''

Unions are finding that fast-food workers are all too happy to protest, figuring they don't have much to lose.

''Marx said, 'Workers of the world unite, you have nothing to lose but your chains,' and when it comes to fast-food workers, they really have nothing to lose,'' Professor Nelson Lichtenstein a labour expert at the University of California, Santa Barbara said.

''There's no career prospects from a fast-food restaurant, high turnover, unpredictable hours.''

Companies such as McDonald's have stood behind their pay practices. ''The story promoted by the individuals organising these events does not provide an accurate picture of what it means to work at McDonald's,'' company spokeswoman Ofelia Casillas said.

At Burger King, more than 99 per cent of the US restaurants were owned by franchisees, who controlled wage decisions for employees, the company said. Workers in the system received compensation and benefits ''that are consistent'' with the fast food industry.

The National Retail Federation called Thursday's protests a ''publicity stunt'', saying they were ''just further proof that the labour movement is not only facing depleted membership rolls, they have abdicated their role in an honest and rational discussion about the American workforce''.

But a letter signed by economists in July supported an increase in the minimum wage, finding that if it were raised to $US10.50, fast-food restaurants would see only about 2.7 per cent higher costs. The eateries could absorb those cost increases by raising menu prices and by allowing low-wage workers to get more of the business's revenue, it said.

Whatever the impact protests have on companies, concerns are growing over income inequality in the US.

While the economy has recovered for households with a net worth of $US500,000 or more, the recession is continuing for other groups, an April Pew Research Centre study shows.

The data, which underscores the nation's growing income inequality, shows that wealthy households boosted their net worth by 21.2 per cent in the aftermath of the recession that ended in June 2009, while the rest of America lost 4.9 per cent.

Congress last voted to raise the minimum wage in 2007, and President Barack Obama's call this year to increase it to $US9 an hour has gone nowhere .

''Inequality has steadily risen over the decades,'' Mr Obama said on Wednesday to commemorate the March on Washington civil-right rally in 1963. ''Upward mobility has become harder.''

Los Angeles Times, Bloomberg