Two explosions, at least one of which officials believe to have come from a suicide bomber, were set off inside the city's main international airport and a third went off at the city's Maelbeek metro station.
Presdent Barack Obama pledged on Tuesday morning that the U.S. will stand in "solidarity" with Brussels following the "outrageous attacks against innocent people."
"We will do whatever is necessary to help our allies in Belgium in bringing justice to those responsible," he declared.
Meanwhile, everyone from Miley Cyrus to presidential candidates Donald Trump and Hillary Clinton have spoken out in support of the city.
Miley Cyrus: "My ❤️ breaks for Brussels today..... My heart breaks for the world today.... When will this end, and the love begin..... This isn't the planet I wanna live on ? NOTHING in the world matters but giving love to others! It's what we need more than anything else! MORE LOVE LESS EVIL!!!"
"Talking with [Happy Hippie Foundation] now about what we can do to be there for our friends in Brussels ... Stay tuned on how to get involved ❤️ today we must focus on what unites us .... not divides us," Miley also wrote on Instagram.
Rihanna: "Praying for all those who've been affected by acts of terror across the globe, most recent being in Brussels...Heart breaking!"
Praying for all those who've been affected by acts of terror across the globe, most recent being in Brussels...Heart breaking! 🙏
Khloe Kardashian: "#PrayForUsAll Terrorism is one of the scariest things. It does not function like anything else in this world. The Dalai Lama once said "love and compassion are necessities, not luxuries. Without them, humanity cannot survive." The world falls to its knees yet again over terrorism ? #PrayForBrussels #PrayForPeace #PrayForUsAll."
Kris Jenner: "Waking up to this heartbreaking news this morning. My thoughts and prayers are with you today, #Brussels."
Donald Trump: "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!"
Hillary Clinton: "My thoughts and prayers are with the families of those killed and wounded, and all the people of Belgium. ...These terrorists seek to undermine the democratic values that are the foundation of our way of life. They will never succeed."
My thoughts and prayers are with the families of those killed and wounded, and all the people of Belgium. -H