The country announced measures for 2026 that will affect pensions, the retirement age and social support, prompting unions to call three days of action.
In Brussels, public transport stopped and Brussels Airport cancelled all departing flights, with many arrivals also scrapped. Tram and bus lines across the country were quiet as workers joined the strike.
Union leaders described the budget as a setback for workers and said the impact would fall on households already under pressure.
Similar walkouts have taken place throughout the year, including a major demonstration in October that drew more than 100000 people.