As Belgium's allies voiced anger and solidarity, Russia and Turkey -- themselves targets of deadly attacks in the last eight months -- said the blasts rammed home the need to fight terrorism of every hue.
Belgian Prime Minister Charles Michel described Tuesday as "a day of tragedy, a black day," while European Union (EU) president Donald Tusk lashed "another low by the terrorists in the service of hatred and violence."
Around Europe, national leaders pledged support for Belgium and many characterised the attacks -- explosions that ripped through Brussels airport and a metro train during rush hour -- and as an assault on liberty itself.
"At this difficult hour, Europe stands up, together and as one. Belgium is not alone," German Foreign Frank-Walter Steinmeier said.
"Our Union's capital is under attack. We mourn the dead and pledge to conquer terror through democracy," the Greek foreign ministry said in a tweet.
It added in French, "Nous sommes tous Bruxellois," -- "We are all citizens of Brussels."
Spanish Prime Minister Mariano Rajoy said: "Terrorism will never defeat us. The union of democrats in Europe will always prevail over barbarism and madness."
Swedish Prime Minister Stefan Lofven described the blasts as "an attack against democratic Europe. We will never accept that terrorists attack our open societies."
Italian Prime Minister Matteo Renzi tweeted: "My heart and spirit in Brussels, Europe," while Interior Minister Angelino Alfano said "the Brussels attacks strike the heart of our Europe."
"The whole of Europe has been hit," French President Francois Hollande declared, urging the continent to take "vital steps in the face of the seriousness of the threat."
In Paris, the Eiffel Tower was to be lit up in the colours of Belgium on Tuesday night in homage to "the victims, their families and the Belgian people," said city mayor Anne Hidalgo.
"We will never let these terrorists win," said British Prime Minister David Cameron. "We face a very real terrorist threat right across the different countries of Europe and we have to meet that with everything we have."
Across the Atlantic, US State Department spokesman John Kirby said "The United States stands with the people of Belgium. We are ready to support the investigation as appropriate."
Canadian Prime Minister Justin Trudeau said: "My thoughts are with the victims as we stand with Belgium and the EU."
Global action on terror
Russia and Turkey said the blasts underscored the global need to fight terrorism across borders.
A Russian plane was downed by a bomb over the Sinai Peninsula in October that killed 224 people, while Turkey has suffered more than 200 civilian deaths in six major attacks since July, blamed on Kurdish rebels and jihadists.
Russian President Vladimir Putin lashed out at what he called "barbarous crimes" and expressed his condolence with Belgium.
"(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."
Turkish Prime Minister Ahmet Davutoglu said the attacks "have once again shown the worldwide character of terrorism."
Foreign Minister Volkan Bozkir added: "Every effort must continue to fight terrorism without distinction and those who support terrorism."
On Friday, President Recep Tayyip Erdogan accused Europe of "surrendering to terror" for letting PKK sympathisers erect a tent outside the European Council building in Brussels last week as EU leaders held a summit with Turkey on migration.
"Despite this clear reality, European countries are paying no attention, as if dancing in a minefield," Erdogan had charged.
Attacks un-Islamic, say clerics
Pope Francis described the attacks as "blind violence, which causes so much suffering."
"Imploring the gift of peace from God, (the pontiff) invokes divine blessings on the bereaved families and the Belgian people," he said in a message to Jozef De Kesel, the archbishop of Mechelen-Brussels.
In Cairo, Sunni Islam's leading seat of learning, Al-Azhar, said the blasts "violate the tolerant teachings of Islam" and urged the international community to confront the "epidemic" of terrorism.
The shockwave of the attacks also reverberated in the US presidential campaign, where Republican frontrunner Donald Trump said the cause of the bloodshed was "no assimilation" by immigrants.
"Belgium is not the Belgium you and I knew from 20 years ago, which was one of the most beautiful and safest cities in the world," Trump told NBC.
"Belgium is a horror show right now. Terrible things are happening. People are leaving. People are afraid. This all happened because, frankly, there's no assimilation."