The warning signs had been building along with the coronavirus case numbers, but the early morning decision to order Brussels residents to wear masks caught many by surprise.

"I don't see why I should wear a mask in a deserted street," declared Sania, a pedestrian in her thirties as news filtered through the city.

The head of the Brussels-Capital regional government, Rudi Vervoort, had already ordered that masks must be worn if the city once again saw more than 50 new cases daily per 100,000 inhabitants.

Federal health officials admit that this represents a fairly arbitrary figure, but Belgium is seeing a rise in infections and officials want to act fast and stifle or slow a second wave.

On Wednesday, the daily number of new infections per 100,000 in the region -- 1.2 million people in 19 communes or municipal districts -- hit 54.4.

Vervoort's order thus came into effect, and Brussels residents are now required to wear masks in all public spaces and all spaces to which the public has access.

They have also been reminded to continue to respect social distancing rules as far as possible while out of their homes, and to restrict themselves to a tight social bubble.

But Belgium has not been spared western Europe's current sweltering heatwave, and in the early hours of the new regulations many passers-by were sceptical.

"When there's nobody around, I don't put my mask on," admitted 67-year-old Marie, heading out to visit her mother with her mask dangling from her hand.

"Once there's a bit of a crowd, I put it on, out of respect for the others."

Marie was at least aware that rules at been tightened. Many more who spoke to AFP had not heard about a measure that had been announced before seven am.

"It doesn't make sense," said Brussels resident Lisa. "I think if you cannot keep the distance, 'OK, I wear the mask'. If I'm alone in the street and there's no way I could have, like, contact with people -- yeah, it's a bit silly."

Belgium as a whole has seen 600 new infections confirmed per day for the past week, but there are wide regional variations.

The northern port of Antwerp was a hot-spot for the epidemic from mid-July, but has since seen the spread slow down markedly and now Brussels is taking its turn as source of concern.

- 'Warning signal' -

"Rising figures don't necessarily mean the situation is severe today, but they represent a warning signal that we need to intervene," said Frederique Jacobs, spokeswoman for the federal COVID task force.

But the epidemiologist Yves Coppieters sees the city-wide mask order as unhelpful and "something of an admission of failure".

He points to the much greater increases in the downtown Bruxelles-Ville commune and the poorer, densely-packed northern and western districts of Anderlecht and Schaerbeek, than in the leafy southern and eastern suburbs.

"It's as if we're already losing track of the transmission clusters," he told RTBF public television. "We should be able to act more locally."

Belgium is a densely-populated country of around 11.5 million with open borders and home to an international population as well as the headquarters of the EU and NATO.

With more than 75,000 cases of COVID-19 and 9,885 deaths to date it is dealing per capita with one of the most deadly outbreaks of the global epidemic.AFP