Multiple explosions rocked Brussels, Belgium on Tuesday morning, killing at least 31 people and injuring more than 100.
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."
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."
Republican nominees Ted Cruz and John Kasich, as well as Democratic nominee Bernie Sanders, weighed in on social media.
European politicians and organizations
Belgian Prime Minister Charles Michel:
The Belgian Royal Family:
("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.
French Prime Minister Manuel Valls said France and Belgium stand together against terrorism.
British Prime Minister David Cameron:
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:
European Council president Donald Tusk:
Peter Altmaier, chief of staff for German Chancellor Angela Merkel:
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.