Landmarks from the Eiffel Tower in Paris to Berlin's Brandenburg Gate were lit up in the black, yellow and red of Belgium's national flag in solidarity.
US President Barack Obama vowed to stand with Belgium in the face of the "outrageous" attacks and ordered US flags flown at half mast, while the FBI and New York police said they would send investigators to help.
UN chief Ban Ki-moon said those responsible for the "despicable" bombings should face justice, while Belgian King Philippe condemned the "cowardly and odious" attacks.
There were chaotic scenes at Brussels airport after the bombers struck around at around 8 am local time (0700 GMT), as plumes of dark smoke could be seen rising from holes punched through the roof of the building by the blasts.
"A man shouted a few words in Arabic and then I heard a huge blast," airport baggage security officer Alphonse Lyoura told Agence France-Presse, his hands bloodied.
"A lot of people lost limbs. One man had lost both legs and there was a policeman with a totally mangled leg."
About an hour after the airport blasts, a third explosion rocked Maalbeek metro station, in the heart of the city's EU quarter, just as commuters were making their way to work.
Paramedics tended to commuters with bloodied faces as the city's normally peaceful streets filled with the wailing of sirens.
'Bombs in their bags'
Pierre Meys, spokesman for the Brussels fire brigade, told Agence France-Presse at least 14 people had been killed at the airport, while Brussels mayor Yvan Mayeur said "around 20" died in the metro.
Among them was Adelma Marina Tapia Ruiz, a Peruvian woman who had been living in Brussels for 6 years and was with her family in the airport when the blast went off, according to the foreign ministry.
More than 200 people were wounded in the two attacks, including 4 Mormon missionaries – 3 Americans and one French – two Britons, two Colombians and an Ecuadoran. France said 8 of its nationals were hurt, though it was unclear if this included the Mormon missionary.
Belgian authorities published surveillance images showing the 3 male suspects of the airport attack. Two had dark hair and were wearing a glove on only one hand, and a third, who is being hunted by Belgian police, was wearing a hat and a white coat.
"They came in a taxi with their suitcases, their bombs were in their bags," Zaventem mayor Francis Vermeiren said.
"They put their suitcases on trolleys, the first two bombs exploded. The third also put his on a trolley but he must have panicked, it didn't explode."
Belgian authorities had been on alert after Abdeslam, Europe's most wanted man, told investigators he had been planning an attack on Brussels.
Last Tuesday, March 15, an Algerian ISIS-linked militant was killed in a shoot-out in the south of the city.
Investigators believe Abdeslam slipped out of the apartment as the gun battle erupted. He was arrested 3 days later in Brussels' gritty Molenbeek district, just around the corner from his family home. – Danny Kemp and Lachlan Carmichael, AFP / Rappler.com