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!"
Reese Witherspoon: "On this day, my heart goes out to all the people of Belgium. #PrayForBrussels"
Kendall Jenner: "My heart goes out to all those in Brussels. Praying for you. Praying for the world."
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."
Nick Jonas: "Praying for peace around the world. Brussels my heart breaks for you."
Priyanka Chopra: "Whats happening to the world? My prayers to the victims and families of the #BrusselsAttacks And my prayers for non-violence #NoWarForPeace."
Rose McGowan: "Oh, #Brussels My heart, and so many others, goes out to you."
Lisa Vanderpump: "This is just awful,34 dead 170 injured..in Brussels."
Elvis Duran: "Healing love #Brussels"
Meghan McCain: "Prayers for everyone in Brussels. There is true evil in this world."
Josh Gad: "Beyond sad and outraged that the scourge of the earth continue to feel the need to kill innocent people everywhere. #Brussels #WeStandWithU"
Lewis Hamilton: "My thoughts and prayers are with all those affected in Brussels ?#prayforbrussels."
Jeb Bush: "Praying for the victims, families and people of #Brussels following today’s brutal terror attacks. We must defeat radical Islamic terrorists."
Ruby Rose: "Devastating news to hear this morning. Senseless acts of violence. When will we stop all of this madness. Heart goes out to Brussels today."
Hoda Kotb and Kathie Lee Gifford: "Our thoughts and prayers are with the people in Brussels on this tragic day."
Dr. Oz: "My heart is with everyone affected by the terrible events in Brussels. These abhorrent attacks aren't just against the city where they occur, they are against all of us. #brussels."
Janelle Monáe: "Peace peace peace peace be with #Brussels. been there on many occasions. Beautiful people. Sending LOVE TO ALL during this time."
Monica Lewinsky: "❤️ to Brussels."