Hundreds of thousands of French workers are expected to join the country's second nationwide strike in two months.
Workers are protesting against President Nicolas Sarkozy's economic policies. Unemployment has reached two million and is expected to rise further. Demonstrations are planned in about 200 towns and cities. Many schools are closed and public transport disrupted.
Organisers predict the protest will be bigger than one in January, when more than a million people took part.
"More coaches had to be booked for the demonstrators for example," Bernard Thibault, the head of France's biggest union, the CGT, said.
"More stoppages and strikes have been decided on in companies, so there will be more people," he added.
Marches by strikers have got under way in Marseille, Lyon and Grenoble, with a major rally due to be held in Paris from 1pm.
The strikes began on Wednesday evening with staff on transport networks. The national rail operator, SNCF, cancelled 40% of high-speed trains and half of regional services. A third of flights out of Paris's Orly airport have been cancelled, while a tenth of France's electricity output has been shut down with workers on strike.
However, buses and the Metro rail system in Paris were running normally, thanks to a new law enforcing a minimum transport service during strikes. But with many schools and public buildings shut for the day, the number of workers travelling into the capital was reduced.
Private-sector firms were also expecting a depleted workforce with staff from the beleaguered car industry, oil and retail sectors taking part in the strike.
The unions say the 26bn euro (£24.5bn) stimulus package for France's struggling economy, unveiled by President Nicolas Sarkozy in December, does not go far enough. A further 2.4bn euros ($3.2bn; £2.3bn) of measures, including tax breaks and social benefits, presented by President Sarkozy after January's strike has failed to placate them. They want him to increase the minimum wage and scrap his plans to cut public-sector jobs.
Recent polls show three-quarters of French people support the strikers. Many commuters on Thursday said they backed the action, but hoped it would be short-lived.
"Fundamentally I agree, but too much is too much," one was quoted as saying. "There are strikes in the transport sector too often and we have to put up with them."
President Sarkozy said on Wednesday that he "understands the concerns of the French people" but has ruled out plans for further measures.
Unemployment is likely to shoot up to 10% in the next 12 months with a further 350,000 lay-offs expected by the end of this year.
Many people are angry that big companies like the oil giant Total are making staff redundant while simultaneously announcing record profits.