"There are already not enough personnel and they want to shed more," Philippe Martinez of the CGT trade union said as the march in Paris prepared to set off.
Civil servants and other public-sector employees are "often called lazy parasites" and "need some respect", he added.
Demonstrations were planned in some 130 towns and cities and had already started in Rennes and Nantes, in western France, Montpellier in the south and Strasbourg in the east.
For the first time in about 10 years all of the trade unions backed the call to strike and demonstrate, angered by the government's plan to shed 120,000 jobs during President Emmanuel Macron's five-year term, freeze basic wages, end payment for the first day of sick leave and raise the CSG payroll tax.
The public sector employs 20 percent of Frence's workforce and the strike has affected crèches, schools, hospitals and other services.
About 30 percent of flights were cancelled because of industrial action by air-traffic controllers.
Noting a "large mobilisation", government spokesman Christophe Castaner promised to "listen to their concerns".
Budget Minister Gérard Darmanin has promised to meet the unions to discuss pay on 16 October and on Tuesday promised that no state employee will suffer a drop in income.
The government says that the job losses will be by natural wastage and will be limited to 1,600 in 2018.
The unions also fear the government plans to privatise services and abolish sector-wide pay scales.
The day of protest comes after two CGT-led strikes in September that failed to stop Macron's labour reform.
Several major unions did not back those demonstrations and the CGT is trying to persuade them to back join action in the coming months.
A meeting on Monday evening failed to agree on joint action but did set a date for a further discussion on 24 October.
Further protests ahead
According to an opinion poll published Monday, 57 percent of the public back the strikes against Macron's labour policy but another poll showed a revival in the president's flagging popularity, despite a series of gaffes in which he was perceived as making derogatory comments about working people.
The government faces further protests from pensioners and other groups of workers, notably lorry drivers and railworkers, as well as discontent in local authorities, which have seen their budgets cut and an important local tax threatened with abolition.