The influential CGT union, which called the marches in Paris and seven other cities, declared that the day was a success.
CGT boss Bernard Thibault said: "We are not naive, the employers are on the offensive. There is pressure on the government to get measures that are favourable for employers and in this situation wage earners cannot remain on the sidelines."
Thibault said "tens of thousands" were on the streets following the union's call.
The port at Le Havre ground to a halt as all 2,400 dock workers went on strike.
Union sources said a majority of the estimated 1,200 shipping agents also stopped work at the country's second terminal container port after Marseille but said passenger services to Britain were unaffected.
The protests come at a time of mounting economic gloom.
The International Monetary Fund on Tuesday slashed its global growth forecast and said France would record only 0.1% GDP growth in 2012, against 0.3% forecast in July.
It said the projected figure for next year was 0.4% — or half of that forecast earlier.
French unions fear that the Socialist government's efforts to plug a €37 billion ($48 billion) hole in public finances will lead to more job cuts.
French unemployment recently breached the three-million mark and crossed 10%.