The Republican front-runner, who has made immigration and security issues central to his 2016 presidential bid, discussed the reported suicide attacks in an interview on Fox News' "Fox and Friends."
"I will tell you, I've been talking about this a long time, and look at Brussels," Trump said. "Brussels was a beautiful city, a beautiful place with zero crime. And now it's a disaster city. It's a total disaster, and we have to be very careful in the United States, we have to be very careful and very vigilant as to who we allow in this country. "
In an another interview, Trump also said he would be "fine" with waterboarding Salah Abdeslam, one of the leaders of the Paris attacks several months ago who was just captured in Brussels, in order to get more information on potential future attacks.
"Well I'm not looking to break any news on your show, but frankly the waterboarding, if it was up to me, and if we changed the laws or have the laws, waterboarding would be fine," the Republican front-runner said on NBC's "Today" show. "I would say they should be able to do whatever they have to do."
"You know, we work within laws. They don't work within laws -- they have no laws. We work within laws. The waterboarding would be fine, and if they could expand the laws, I would do a lot more than waterboarding," he said.
Trump has vacillated in his campaign about whether or not he would pursue using waterboarding -- which is considered torture and thus illegal. After getting criticized by former national security officials, Trump said he would instead seek to expand the laws, presumably to make such tactics legal.
On Fox, he described Brussels now as an "armed camp."
"If you went into Brussels 20 years ago, it was like a magical city. Now you look at it, it's an armed camp," Trump said. "You want to lead your life, you don't want to be living in an armed camp for your whole life. And there is a certain group of people that is making living a normal life impossible."
Trump was quick to link the attacks to Muslim refugees and migrants from the Middle East who have flooded into Europe as a result of Syria's civil war and turmoil in the region.
"It's going to get worse and worse. In my opinion, this is just the beginning. It will get worse and worse because we are lax and we are foolish -- we can't allow these people, at this point we cannot allow these people to come into our country. I'm sorry," he said. "This is a story that just seems to be more and more happening and it's really not very pretty to watch."
Europeans, Trump warned, need to change their tactics.
"Those countries better get smart fast, because they're just disintegrating," he said.
Trump also took the opportunity to criticize "liberal policies" that he blamed for the tension and fear surrounding the refugee crisis.
"We're not the victims here -- we're acting like this is our fault," he said. "That's the problem with the liberal policies of this country and this world, it's acting like it's our fault. It's not our fault, okay, it's not our fault. It's their fault. And they have to come out and they have to say, hey look, this is happening."
Clinton, Sanders react to Brussels
Democratic front-runner Hillary Clinton told NBC's "Today" show the U.S. has to be "absolutely smart and strong and steady in how we respond."
"We've got to stand in solidarity with our European allies," she said.
But the former secretary of state cautioned against blanket bans on immigrants based on the attacks.
"It's unrealistic to say we're going to completely shut down our borders to everyone," she said. "I know that Americans have every reason to be frightened by what they see, (but) we've got to work this through, consistent with our values."
Clinton said in a statement that "the people of Brussels, of Europe and of the world will not be intimidated by these vicious killers."
"Today's attacks will only strengthen our resolve to stand together as allies and defeat terrorism and radical jihadism around the world," she said.
She later tweeted, "My thoughts and prayers are with the families of those killed and wounded, and all the people of Belgium" and "These terrorists seek to undermine the democratic values that are the foundation of our way of life. They will never succeed. -H"
Vermont Sen. Bernie Sanders called the attack "another cowardly attempt to terrorize innocent civilians."
"Today's attack is a brutal reminder that the international community must come together to destroy ISIS. This type of barbarism cannot be allowed to continue," he said in a statement.
He also tweeted, "We offer our deepest condolences to the people of Brussels and stand with our European allies to offer any necessary assistance."
Speaking with reporters later in the day, Sanders said, "We have to have significantly improved intelligence, and that intelligence cannot be done just within the United States ... We have to share with countries around the world, to monitor the people who would do us harm. That goes without saying."
Asked about calls to monitor mosques following the attacks, Sanders said, "It would be unconstitutional, it would be wrong. We are fighting a terrorist organization, a barbaric organization that is killing innocent people. We are not fighting a religion."
Rest of GOP field
Ohio Gov. John Kasich said he was "sickened by the pictures of carnage."
"The wave of terror that has been unleashed in Europe and elsewhere around the world are attacks against our very way of life and against the democratic values upon which our political systems have been built," he said in a statement. "We and our allies must rededicate ourselves to these values of freedom and human rights. We must utterly reject the use of deadly acts of terror."
Later, Kasich called on President Barack Obama, currently on a foreign trip to Cuba, to return to the U.S.
"The president must return home immediately and get to work with our allies to respond with strength against the enemies of the west," he tweeted.
In a news conference in Minneapolis later in the day, Kasich said the U.S. would have to reassemble a global coalition of European and Arab allies like the one from the first Gulf War to "destroy ISIS."
"The challenge I think we are always going to face, especially here in the United States, is the issue of the lone wolf or lone wolves, small groups of people who are intent on inflicting harm," he said. "We have seen it in a number of ways here in the United States, most recently in San Bernardino."
Kasich also cautioned against sweeping targeting of Muslims, saying that would only lead more to become radicalized and unfairly target innocent people.
Though the attackers have yet to be identified, Texas Sen. Ted Cruz called the blasts "just the latest in a string of coordinated attacks by radical Islamic terrorists perpetrated those who are waging war against all who do not accept their extreme strain of Islam," in a Facebook post.
"Radical Islam is at war with us," he said, and accused Obama of refusing to "acknowledge this reality."