STORY: Unions said around 80,000 people had joined the protest. Train services were running, partly to allow protesters to converge on the capital.
Brussels Airport said it could not allow passenger flights to depart because the industrial action extended to security personnel. Local public transport was running minimal services.
Inflation hit 9% in June in Belgium, mirroring sharp price rises elsewhere largely because Russia's invasion of Ukraine has hit grain supply and caused the cost of energy to spike.
Prime Minister Alexander De Croo said Belgian workers were better protected than their counterparts in most other European Union countries because wages were indexed to inflation.
He told public broadcaster RTBF the government had extended measures to reduce sales tax on gas, electricity and fuel until the end of the year.