The UN secretary-general called them "despicable attacks"
Following a series of explosions at a Brussels airport and metro station, the world is reacting in solidarity.
Speaking during his historic visit to Cuba, US President Barack Obama says the world must unite against terrorism.
The United Nations Secretary-General Ban Ki-moon says: "The despicable attacks today struck at the heart of Belgium and the centre of the European Union", adding that those responsible will be brought to justice.
Other senior UN officials have strongly condemned the terrorist bombings, extending condolences to the victims and their families - while expressing solidarity with the people and government of Belgium.
In France, the Eiffel tower lit up on Tuesday evening in the colours of the Belgian flag, as a gesture of solidarity with the country in the wake of the attacks.
Elsewhere, other monuments around the world have followed suit, including the Brandenburg Gate in Berlin and the Trevi Fountain in Rome.
Caretaker Taoiseach Enda Kenny says "Europe is under attack" after this morning's fatal blasts.
Once again Europe is under attack. We stand with Belgium. Those using death and violence must and will be defeated.— Enda Kenny (@EndaKennyTD) March 22, 2016
Expressing his solidarity with the Belgian people, Mr Kenny writes: "Those using death and violence must and will be defeated".
The President Michael D Higgins stated that he is "deeply saddened" at "the tragic loss of lives" in Brussels.
"These attacks strike at the fundamental right of all to live in peace. These actions must not undermine the will of all Europeans to live and work together".
"I wish to express my sincerest condolences to the families of all those bereaved and affected by today's tragedies. All of our thoughts are with the people of Brussels at this time of tragedy".
President Higgins says he has conveyed the sympathy of the Irish people to King Philippe.
We have increased our security setting for Belgium to exercise extreme caution. Latest updates on our website: https://t.co/FR0Idm86a1— DFATravel (@dfatravel) March 22, 2016
The Department of Foreign Affairs has advised Irish citizens that extreme caution should be exercised by those travelling to Belgium.
Those who are concerned about friends and family in Brussels are advised to call the department's Consular Division on +353-1-418-0200.
The acting Foreign Affairs Minister Charlie Flanagan told Newstalk Lunchtime this was an attack "on the European institutions".
British Prime Minister, David Cameron, says he is "shocked and concerned".
I am shocked and concerned by the events in Brussels. We will do everything we can to help.— David Cameron (@David_Cameron) March 22, 2016
He said the UK "will do everything we can to help" Belgium, as security was stepped up at airports including Heathrow and Gatwick in response to the apparent terrorist attack.
Mr Cameron confirmed he would be chairing a meeting of the British government's COBRA emergency committee to discuss events in Brussels.
While President of the European Council, Donald Tusk, says the attacks "mark another low by the terrorists in the service of hatred and violence".