The mask-wearing regulation is one of the strictest currently applied in European capitals and follows a similar decision by authorities in Madrid in July
Wearing a face mask became compulsory on Wednesday in all public places in Brussels as the number of Covid-19 infections rose to a government alert level that puts the city among the worst affected in Europe.
The Belgian capital, which hosts the headquarters of the European Union and NATO, recorded on average 50 cases per 100,000 inhabitants daily over the last week.
Everybody in the city, which has a population of 1.2 million, now has to wear a face mask when in parks, on streets or in any other public sites, as well as in private space accessible to the public, the regional government said.
Until now masks had been compulsory only in crowded public spaces and enclosed sites, such as shopping malls.
The mask-wearing regulation is one of the strictest currently applied in European capitals and follows a similar decision by authorities in Madrid in July. In Paris face masks are mandatory outdoors only in crowded places.
Belgium has been hit hard by the pandemic, with nearly 10,000 deaths linked to the coronavirus so far, which gives the country of 11 million one of the world's highest death rates from Covid-19 per head.
Infections have been rising steadily in recent weeks and it now has one of the highest number of cases per inhabitants of any European country.
On Tuesday the country had on average more than 60 daily cases per 100,000 inhabitants over the last two weeks, compared to 7.5 in Italy, 13 in Germany, 29 in France and 90 in Spain, according to figures collected by the European Union's public health body, the European Centre for Disease Prevention and Control.
Last week the British government said it would impose a quarantine on people arriving from Belgium.
Children below 12 years, people doing sport or physical works in public places are exempt from wearing a mask.