More than 150 demonstrations against the centre-right government's First Employment Contract (CPE) were staged in cities across the country on Saturday, the largest in the capital drawing tens of thousands of students, workers, pensioners and families.
Claiming a turnout of 300,000 in Paris and a million nationwide, union leaders claimed success and urged Dominique de Villepin to heed the message from the street.
Bernard Thibault of the CGT union said: "The demand for withdrawal of the CPE is gathering ever greater force. Seventy per cent of the French want it withdrawn, and 80% of young people. The government is in a dead-end."
Bruno Julliard of the UNEF students' union said: "Today we can clearly see that the mobilisation is stronger than ever. Either the government listens to reason and withdraws the CPE, or it will be obliged to do so next week - because we will be back in the street."
The FIDL high-school students' union promised a new day of action on Thursday if the government did not back down.
Several leaders of the opposition Socialist Party including Laurent Fabius and Dominique Strauss-Kahn joined the Paris demonstration, which left Denfert Rochereau Square in the south of the capital in the early afternoon.
Riot police were stationed discreetly in lines of buses near the course of the route to Nation Square on the other side of the river Seine, but the march took place in a good-humoured atmosphere.
"Today we can clearly see that the mobilisation is stronger than ever. Either the government listens to reason and withdraws the CPE, or it will be obliged to do so next week - because we will be back in the street"
UNEF students' union
An opinion poll on Friday showed that 68% of the public oppose the youth jobs plan, up from 55% just over a week ago.
The campaign of opposition has developed into a serious political crisis for the 52-year-old prime minister, who has made implementation of the CPE a personal mission but has been criticised - from even within his own Union for a Popular Movement (UMP).
De Villepin has offered to add new "guarantees" to sweeten the pill and called for "dialogue", but student and union leaders say they will not enter talks unless the CPE is first abandoned.
University chiefs who met de Villepin on Friday night said that the prime minister appeared willing to make new concessions.
Jacques Chirac, the president, has so far stood by de Villepin - backing the CPE and calling for negotiations - but commentators said he would be keeping careful watch on the scale of the protests.