Coinciding with the debate, protesters against record unemployment staged the latest in a series of demonstrations in Paris and other cities.
An estimated 10,000 people took to the streets in support of Prime Minister Jospin's 35-hour week initiative and calling for higher welfare benefits.
Martine Aubry, the French Employment Minister, laid out the government position at the start of what promises to be a crucial debate.
"I'm convinced...that shortening the working week, if carried out in a well-organised, decentralised manner and by negotiation, can create jobs, lots of jobs," she said.
Prime Minister Lionel Jospin sees the disputed reduction of four hours from the working week as the solution to unemployment that has eluded French governments for years.
But his reform package was swamped with some 1,500 amendments from opponents on the first day of debate.
Right-wing opposition parties, backed by France's employers, contended that the bill would force companies to look abroad for cheaper labour, hampering competitiveness and thereby lengthening France's already extensive dole queues.
The unemployed protesters' demands for higher welfare benefits have confronted Prime Minister Jospin with his first major crisis after eight months of power.
His supporters hope that the bill will calm the militant jobless movement and boost Mr Jospin's popularity, which has been affected by the series of protests launched by unemployed activists.
Although the fourth national action day of protest rallies drew slightly fewer people than previous protests, activists were not downhearted.
"Our movement is far from dead. They have the money, we have the time," said one jobless leader, Claire Villiers.
Unemployment is the political issue of the moment in France, with discontent over record jobless levels of over 3m - a staggering 12.4% of the workforce.
Mixed feelings in France on 35-hour week
(27 Jan 98 | World)
Unemployment debate opens in French Assembly
(28 Jan 98 | World)
Jospin plans cut in working week
(26 Jan 98 | World)
Jospin announces one billion-franc emergency fund for jobless
(09 Jan 98 | World)
Premier Ministre et Gouvernement Français
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