The Islamic State group, also known as ISIS, has claimed responsibility, according to the Associated Press. The group claimed that its extremists had detonated suicide belts at the site of the explosions. The city is now on its highest terror alert level.
As world leaders learned of the attacks, they expressed their condolences with the victims and spoke about the importance of combating terrorism. Presidential hopefuls in the United Sates spoke about what they would do if elected.
President Barack Obama, who is on his historic trip to Cuba, spoke about the attacks Tuesday morning.
"This is yet another reminder that the world must unite," he said. "We must be together, regardless of nationality or race or faith, in fighting against the scourge of terrorism."
U.S. presidential candidates
Republican nominee Donald Trump told Good Morning America he would closely monitor people entering the United States if elected. He also criticized the security situation in Europe.
"The police don't have control of it anymore. It's a very sad thing to see what's happening," he said of the European cities. On efforts to combat terrorism, he said, "If they're not going to play by the rules and we're going to play by the rules, we won't have too many victories."
Do you all remember how beautiful and safe a place Brussels was. Not anymore, it is from a different world! U.S. must be vigilant and smart!— Donald J. Trump (@realDonaldTrump) March 22, 2016
Democratic nominee Hillary Clinton also spoke to Good Morning America. She said she too would strengthen security, but said that "closing our borders to all people of a certain religion" isn't consistent with American values.
"We will defeat terrorism and radical jihadism, but we are going to have to really bear down on what works and how best to implement that," Clinton said. "We've got to strengthen our resolve, to stand together."
My thoughts and prayers are with the families of those killed and wounded, and all the people of Belgium. -H— Hillary Clinton (@HillaryClinton) March 22, 2016
These terrorists seek to undermine the democratic values that are the foundation of our way of life. They will never succeed. -H— Hillary Clinton (@HillaryClinton) March 22, 2016
Republican nominees Ted Cruz and John Kasich, as well as Democratic nominee Bernie Sanders, weighed in on social media.
We offer our deepest condolences to the people of Brussels and stand with our European allies to offer any necessary assistance.— Bernie Sanders (@SenSanders) March 22, 2016
Heartfelt thoughts & prayers are with the victims, their families & all affected by the terror attacks in Brussels. https://t.co/vgRqT8T2HB— Nancy Pelosi (@NancyPelosi) March 22, 2016
I pray for the victims and their families in Belgium. In the face of evil we are united with our Belgian and European friends.— Kevin McCarthy (@GOPLeader) March 22, 2016
European politicians and organizations
Belgian Prime Minister Charles Michel:
I strongly condemn these hateful attacks. Our thoughts go out to the victims and their families. We stand united against terrorism.— Charles Michel (@CharlesMichel) March 22, 2016
The Belgian Royal Family:
Geschokt door gruwelijke aanslagen in Brussel-Nationaal en Brusselse metro. Onze gedachten gaan naar slachtoffers, families en hulpdiensten.— Belgian Royal Palace (@MonarchieBe) March 22, 2016
("Shocked by horrific attacks in Brussels national and Brussels metro. Our thoughts go to victims, families and emergency services.")
French president Francoise Hollande told the Associated Press that "terrorists struck Brussels but it was Europe that was targeted - and all the world that is concerned." He expressed similar sentiments on Twitter, along with solidarity with the Belgian people.
J’exprime mon entière solidarité avec le peuple belge. À travers les attentats de Bruxelles, c’est toute l’Europe qui est frappée.— François Hollande (@fhollande) March 22, 2016
French Prime Minister Manuel Valls said France and Belgium stand together against terrorism.
13 novembre à Paris, 22 mars à Bruxelles, la France et la Belgique ensemble face au terrorisme. pic.twitter.com/OlunclTWRa— Manuel Valls (@manuelvalls) March 22, 2016
British Prime Minister David Cameron:
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
European Union foreign policy chief Federica Mogherini abruptly ended a news conference after she was overcome with emotion. Mogherini gave the conference from Jordan, where she is visiting, with Jordanian Foreign Minister Nasser Judeh.
King of Jordan:
His Majesty King Abdullah II condemns the terrorist attacks in Brussels and offers his condolences to King Philippe of Belgium #Jordan— RHC (@RHCJO) March 22, 2016
European Council president Donald Tusk:
EU returns Brussels solidarity now & will help Brussels, Belgium & Europe counter terror threat https://t.co/khlX0qUvqb— Donald Tusk (@eucopresident) March 22, 2016
Peter Altmaier, chief of staff for German Chancellor Angela Merkel:
All solidarity now with EU, Belgium, #Brussels! Terrorists will never win: Our European values much stronger than hate, violence, terror!— Peter Altmaier (@peteraltmaier) March 22, 2016
Officials with the Eiffel Tower said the landmark will light up in Belgium's colors Tuesday night in solidarity with the victims.
The Associated Press contributed to this report.