'Brussels I love you': Hundreds of residents gather for vigil after terrorist attack

"Brussels I love you" says the message written in chalk on a historic city square, a place for raucous celebration that has become a scene of grief after Belgium's worst terror attack.

Wrapped in the national flag and carrying candles and flowers, Belgians flocked in their hundreds to the Place de la Bourse in the ancient heart of the city to grieve for the dead.

The symbolic heart of the European Union was left reeling Tuesday after some 35 people were killed in bombings at Zaventem Airport and on a metro train.

A lone musician played a cello as a mourner waved a banner reading "United against hate" and another message scrawled on the ground said: "Christians + Muslims + Jews = humanity".

"It's important to get together after moments like these," Leila Devin, 22, told AFP. "It shows we're united against terror."

Belgian Prime Minister Charles Michel joined the mourners after dark fell and European Commission president Jean-Claude Juncker also paid homage to the dead.

"Tonight I am Belgian," he said, full of emotion.

Landmarks around the world, from New York's One World Trade Center to the Eiffel Tower in Paris were lit up in the black, yellow and red of Belgium's national flag in solidarity.

In London, pictures on social media showed the O2 stadium lit up by mobile phone screens as popstar Adele asked fans to "take a moment for Brussels".

Near Brussel's Place de La Bourse square, the usually bustling pedestrian streets of the city centre were nearly empty, with many shops closed.

A stone's throw away, there were no visitors by the famous Manneken Pis statue of a young boy urinating, usually surrounded by a crowd of tourists waiting in line to snap a picture.

"It's sad, it's unfortunate, it's shocking," said Sofiane, an Algerian student, who had come to pay her respects.

- 'We're not scared' -

All day, the sound of police and ambulance sirens echoed through the streets, with roads cut off by heavily armed soldiers and police officers.

As night fell some signs of normality returned, with trains running from the main station and some roads opening up.

But on the square, where the country traditionally celebrates the victories of its "Red Devils" footballers, the crowd continued to swell.

Like Belgium's football team, the attacks have sparked a rare moment of unity in a country that is normally deeply divided between its French and Flemish-speaking communities.

"My mother and I came to show we're proud of being Belgian and that we're not scared. Because this morning I was terrified," said Analphia Desmet, a 22-year-old communications student.

As more and more flowers were placed on the square, the crowd joined together to sing John Lennon's "Imagine".

Mourners waved banners saying "Brussels is beautiful" and "Je Suis Bruxelles" (I am Brussels) -- a reference to the slogan that became a rallying cry after the attacks on Paris's Charlie Hebdo magazine in January 2015.

"We're here to say we're not scared, there's a dozen of them, but we are thousands," said Belgian student Juliette.