The unions fear the government's proposed savings package will hit workers, pensioners and low-income households. They oppose potential changes to wage indexation, VAT and other measures affecting purchasing power, arguing that wealthier people should contribute more.

Students also joined the march to protest against education cuts, rising fees and reforms they fear could make university less affordable.

The demonstration began with crowds carrying placards, playing music and dancing through the capital. But violence later erupted near Brussels-Midi railway station, where some protesters threw objects and fireworks at police.

Officers responded with water cannon and tear gas, while some demonstrators hurled gas canisters back at them. Police deployed additional units equipped with tear gas launchers, supported by a helicopter.

Further unrest was reported near Brussels-Central station. Police said the clashes took place on the sidelines and were separate from the main trade-union march.

Police initially reported 21 people detained, including 13 minors, but the number rose to more than 100 by Friday evening, with around 30 judicial arrests and 80 administrative detentions. No confirmed casualty figures were available, despite clashes, fires and property damage. Police said the main trade-union march remained peaceful.