Police estimated about 300,000 private and public sector workers joined the protests across France on Saturday. The leading Confederation Generale du Travail (CGT) union put the figure at 600,000.
"Chirac, Raffarin, are you sleeping?" demonstrators chanted, venting their anger at French President Jacques Chirac and Prime Minister Jean-Pierre Raffarin.
The French government says the reforms are vital to make the country more competitive in the world economy.
Parliament is debating a government plan to allow staff to increase overtime and work up to 48 hours a week, the maximum allowed under European Union law, instead of the present 35. But managers must first agree to the changes with unions.
Raffarin says rules must be relaxed to help cut high unemployment, now close to 10%, and make the world's fifth largest economy more competitive.
Protesters could hurt France's
EU referendum this year
He is wary after voters punished his government in regional and European parliament elections last year over economic cutbacks, but has played down any parallels with street protests before the last conservative government was voted out in 1997.
Although no elections are due before 2007, the government is concerned French voters could express their anger over reforms when they vote in a referendum on the EU constitution this year.
Both Raffarin and Chirac have urged voters to approve the treaty on its merits and not allow themselves to be sidetracked by domestic political issues.
"The government would do well not only to hear but to listen to the workers," CGT Secretary-General Bernard Thibault said at the start of a rally in Paris.
Four of France's five major unions called the protests, saying the reforms would sound the death knell of the 35-hour week and result in longer hours without extra pay.
The 35-hour week was introduced in 1998 by the previous socialist government in an effort to reduce joblessness. The Socialist party has called on Raffarin to abandon the reforms and reopen negotiations with unions.
"This reform will have very unfortunate consequences on the labour market as ... overtime will be increased which will deprive even more people of jobs," Socialist party leader Francois Hollande said at a demonstration in the western city of Rennes.
A recent poll showed 77% of workers wanted to keep their week at the 35-hour level. Only 18% wanted to work longer hours.