Three blasts ripped through Brussels airport and a metro station, leaving mangled bodies and chaos in their wake.
The suicide attacks come days after the arrest of Paris attacks suspect Salah Abdeslam in the Molenbeek neighbourhood of Brussels.
British Prime Minister David Cameron said:
Swedish Prime Minister Stefan Lofven also condemned the attacks on Twitter, saying: "This is an attack against democratic Europe. We will never accept that terrorists attack our open societies."
Russian Prime Minister Dmitry Medvedev condemned the suicide attacks:
Narendra Modi, India's prime minister, sent his condolences to the victims' families:
Dutch Prime Minister Mark Rutte said that Belgium had "again been hit by cowardly and murderous attacks. Our hearts go out to the victims and next of kin… the Netherlands stands ready to help and support our southern neighbours in any possible way."
One Israeli government minister, however, thought it appropriate to blame the deaths on EU politicians for labelling goods from illegal Israeli settlements - a "distraction" from fighting terrorism.
"Many in Europe have preferred to occupy themselves with the folly of condemning Israel, labeling products, and boycotts. In this time, underneath the nose of the Continent’s citizens, thousands of extremist Islamic terror cells have grown," wrote Science, Technology and Space Minister Ofir Akunis on Facebook.
"There were those who repressed and mocked whoever tried to give warning. There were those who underestimated. To our sorrow, the reality has struck the lives of dozens of innocent people."
Social media users in the Arab world used the Arabic-language hashtag #Belgium to show their solidarity with the victims.
"May God curse the people who have tarnished the image of Islam and Muslims. Islam is a religion of love, tolerance and brotherhood," said Twitter user Mohammad al-Qahtani.
Nasser al-Sahli said: "Even though Belgium is a peaceful country that has nothing to do with war and conflict it wasn't safe from the evil of terrorism. May God be with them."
Rashid al-Faiz apologised to the victims: "I am sorry Belgium for these attacks that were perpetrated by fellow Muslims. We are ashamed and disgraced by what they have done."
Another user said: "IS is leaving Russian, Iran and Israel alone and targeting poor Belgium. My heart goes out to the families of the victims".
Ireland-based Islamic theologian Shaykh Umar al-Qadri also gave his condolences:
Some prominent figures, however, used the attacks to promote their political ideologies.
Republican presidential candidate Donald Trump said:
British columnist Allison Pearson posted:
Dutch politician Geert Wilders, meanwhile, said: "We have to close our borders immediately, send back the Western jihadists from the Syrian war and preemptively arrest and imprison. The safety of the Netherlands comes first!"
The British anti-immigration far-right party UKIP also wasted little time putting out a statement:
Their comments have also been widely condemned and derided: