The strike that began at airports and train stations late Sunday also threatened to halt bus, tram and metro services as well as hit schools, businesses, factories and government offices throughout the country later Monday.
The protest is the climax of a union movement involving the Christian CSC, Socialist FGTB and liberal GGSLB that began last month with a march of more than 100,000 people and violent protests in the capital Brussels.
Unions went ahead with the strike after the right-of-centre government of Prime Minister Charles Michel refused to budge on plans to save 11 billion euros ($13.7 billion) over five years.
French-speaking Michel, who at 38 is Belgium's youngest prime minister since 1840, heads a government coalition of three Flemish-speaking right-leaning parties and his own French-speaking liberals.
The formation of the government in October, five months after elections, was meant to bring some calm to a nation deeply divided between the richer Flanders and the poorer French-speaking Wallonia.
But Belgian unions oppose a decision by Michel's coalition government to scrap plans for a usually automatic cost-of-living raise next year. They also reject public sector cutbacks and plans to raise the retirement age from 65 to 67 from 2030.
The strike will affect not only Brussels but also French-speaking towns like Liege and Tournai as well as Flemish-speaking cities like the tourist hotspot of Bruges and the ports of Antwerp and Ostend.