Transports are affected, including the high-speed international trains, and the main Brussels railway stations are closed. Other public transport in Brussels is very limited, with no tramways, buses or metros in circluation.
Postal staff are also on strike, as are port workers at Anvers, one of the biggest ports in Europe.
Belgian trade unions are protesting government plans aimed at saving 11.3bn euros ($14.8bn), including a proposal to raise the effective retirement age.
EU leaders have gathered in Brussels for thier first summit this year. They will continue talks on the eurozone crisis and are expected to sign a treaty that will bind eurozone states to tougher budget rules known as the "fiscal compact".