Global landmarks from the Eiffel Tower in Paris to Berlin's Brandenburg Gate were lit up in the black, yellow and red of the Belgian national flag in solidarity.
In Brussels, hundreds crowded into Place de la Bourse in the capital's historic centre to grieve for the dead, while in London fans of Adele lit up the O2 stadium with their phones after the pop star asked them to "take a moment for Brussels".
The EU said the Brussels attacks were an assault "on our open democratic society" at a time when the bloc is already on edge after a wave of jihadist violence.
"This latest attack only strengthens our resolve to defend the European values and tolerance from the attacks of the intolerant. We will be united and firm in the fight against hatred, violent extremism and terrorism," leaders and its institutions said in a statement.
More From This Section
"Our Union's capital is under attack. We mourn the dead and pledge to conquer terror through democracy," the Greek foreign ministry said on Twitter. "Nous sommes tous Bruxellois" -- "We are all citizens of Brussels."
Belgian colours lit up the Eiffel Tower in Paris, and hundreds joined a vigil in support of the Brussels victims. Flags were to fly at half mast in France, a nation still raw from last year's jihadist rampage.
German Chancellor Angela Merkel vowed that "the horror is as boundless as the determination to defeat terrorism" and British Prime Minister David Cameron vowed: "We will never let these terrorists win."
US President Barack Obama branded the attacks "outrageous", calling on the world to stand "together regardless of nationality or race or faith in fighting against the scourge of terrorism".
UN Secretary General Ban Ki-moon said he was confident
"Europe's commitment to human rights, democracy and peaceful coexistence will continue to be the true and lasting response to the hatred and violence of which they became a victim today".
He told the ABC: "Australia is allied with Belgium in this battle just as our forebears were 100 years ago in the fields of Flanders, in the First World War."
Turkey, which has seen hundreds killed a wave of bombings blamed on Islamic State as well as Kurdish rebels, said the attacks in Brussels rammed home the need to combat terrorism of every hue.
"The terrorists who targeted Brussels... Are showing once again that they respect no value nor any human and moral limit," President Recep Tayyip Erdogan said in a statement.
"(They) demonstrate once again that terrorism has no borders and threatens people around the world. Fighting this evil calls for the most active international cooperation."
Religious leaders also spoke out against the attacks, which Pope Francis described as "blind violence, which causes so much suffering".
In Cairo, the leading seat of learning in Sunni Islam, Al-Azhar, said the blasts "violate the tolerant teachings of Islam" and urged the international community to confront the "epidemic" of terrorism.
"This all happened because, frankly, there's no assimilation.