BRUSSELS (AFP) - At the Gare du Midi train station in Brussels, a line of taxis awaits desperate travellers facing a city on lockdown, many from London or attack-hit Paris.

The terror alert is at its highest in the Belgian capital following the Paris atrocities and the subsequent raids that have targeted dozens of militant suspects from this city.

"Eighty euros in an hour, as much as yesterday!" said one driver, his smile at odds with the sombre atmosphere.

Brussels city closed its metro system and shopping centres after reports of an "imminent threat" of a gun and bomb attack similar to the horror seen in Paris last week which left at least 130 people dead.

Belgian Prime Minister Charles Michel said authorities feared a "Paris-style" attack "with explosives and weapons at several locations" despite the hundreds of soldiers patrolling the city, home to the EU and Nato.

Arriving at dawn by train on Saturday, many travellers stumbled upon a shuttered metro, likely to remain closed until at least Sunday afternoon.

"By order of the police, the station is closed," 16-year-old Sarah, sporting a military jacket and a lip ring, read from a sign.

She said she had just arrived by train from Antwerp.

"I need to get back home, how am I supposed to do that now? There is no bus that can take me," she said in frustration.

"All they do is scare us, it's not normal."

Jonathan, 26, pulled out his smartphone and snapped a photo of the sign.

"I'm going to post this on Facebook to warn my friends because everybody doesn't really seem to be up to date."

Standing next to an exit, Gilles, an agent for the city's public transport system, waved his arms in all directions as he answered passengers' persistent questions.

"How do we get to Louise?" someone asked.

"Bus 27 and walk the rest of the way!" he responded for the third time.

"You have to be very patient. Some react well, others are very unhappy," he said.

"My role is to give as much information as possible."

The city's historic Grande Place, usually bustling with tourists, is quiet.

Some stragglers traverse the cobblestones to take photos in front of the town hall's gothic architecture.

An armoured vehicle patrols the entrance. Officers step aside to let a bride pass, her white dress a stark contrast against the khakis worn by the guards.

"We'll be quick," a guest murmurs.

The shopping district a few blocks away usually attracts 44,000 people a day but the large Galeria Inno shopping centre is closed.

Management at the neighbouring mall City 2, decided to close at noon.

But during the two hours it was open, shops were deserted, with most residents following the recommendations of the authorities to avoid commercial hubs.

"It's completely empty!" said 26-year-old Adele, who works for a chocolate shop.

"There won't be anybody today, everybody is sharing info on social media," she said.

A bit further on a woman waits for her colleagues to open an accessories boutique.

"It's complicated with the metro (which connects directly to the mall)," she said.

"For safety reasons, I would have preferred to stay warm at home but at the same time we can't give in to panic.

"Paris also has a lot of shops, and they didn't close after the attacks," she said.