Prime Minister Malcolm Turnbull has called the attacks "cowardly" as officials continue to investigate whether any Australians were caught up in the explosions.
Shortly after a series of blasts that killed 34 people in the Belgian capital came a flood of grief and support from leaders across the world.
Speaking to Channel Nine, Mr Turnbull says Australia stands with the people of Belgium.
"Australians condemn these cowardly attacks in Brussels and the poeple of Belgium, our thoughts and prayers are with them. But above all, our most resolute solidarity. We are utterly united, completely united, in the fight against terrorism and the fight against this sort of cowardly violence."
Mr Turnbull says there's no indication that any Australians were hurt in the attacks and ASIO advice says there's no requirement to increase any alert levels in Australia.
Just months after Paris was rocked by a series of violent attacks that killed more than 130 people, French President Francois Hollande pledged his support during Belgium's time of need.
"Terrorism has hit Belgium, but it was Europe that was the target and it is the whole world that is concerned."
While Paris bathes the Eiffel tower in the colours of Belgian flag, residents in the German capital are laying flowers in front of the Belgian embassy.
In her condemnation of the attacks, German Chancellor Angela Merkel stressed the importance of Europe showing a united front.
"The perpetrators are enemies of all the values Europe stands for today. We together as members of the European Union declare our allegiance with great pride, to the values of freedom, democracy and the peaceful coexistence as confident citizens."
Calls for unity also came from outside Europe.
During his historic visit to Cuba, US President Barack Obama says the attacks remind the entire world that terror can strike anywhere.
"We must be together regardless of nationality, or race, or faith, in fighting against the scourge of terrorism. We can and we will defeat those who threaten the safety and security of people all around the world."
Those sentiments are shared by British Prime Minister David Cameron.
"We absolutely stand with them at this very difficult time. These were attacks in Belgium, they could just as well be attacks in Britain, or France, or Germany, or elsewhere in Europe. We must stand together against these appalling terrorists and make sure they can never win."
Canadian Prime Minister Justin Trudeau's remarks have been scathing, saying he and his people are shocked and outraged.
"What happened today in Brussels was an act of terror. It was violence directed at innocent civilians, and its goal was to take lives and instil fear. This cannot and will not be tolerated. Canada strongly condemns these cowardly attacks and stands with Belgium at this most difficult time."
Russian President Vladimir Putin says the attacks underline the need to pool international resources combating terrorism.
"I should begin with expressing condolences to the injured and the families of the victims. We have repeatedly discussed the issues related to the fight against terrorism, and it's possible to efficiently combat it only by united efforts."
But amid so many calls for strength and solidarity, Italian Prime Minister Matteo Renzi has taken a different approach.
As much of Europe struggles to cope with a huge influx of migrants and refugees from Africa and the Middle East, the attacks have intensified calls for the EU to approve complete border closures.
But Mr Renzi says that isn't a credible solution.
"Those who are promising miraculous solutions are unaware of how long this story will be, of how difficult this will be, and are oblivious that their solutions are superficial. Those who are deluding themselves, shouting 'let's close the borders' are not aware that our enemies are already inside our cities."
NATO Secretary-General Jens Stoltenberg says he will continue to monitor the situation closely, on what he's called a "dark day" for Belgium and the world.
"This is a cowardly attack. An attack on our values and on our open societies. Terrorism will not defeat democracy and take away our freedoms."
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