Populist politicians and parties have seized upon the Brussels attacks to push their anti-immigration arguments.
In the UK, the UK Independence Party jumped on the attacks to highlight what he perceives as weaknesses in the European Union.
Mike Hookem, the party’s defence spokesman, released a statement within two hours of the attacks saying the “horrific act of terrorism shows that Schengen free movement and lax border controls are a threat to our security”.
Meanwhile, Ukip's leader, Nigel Farage, said "I'm very upset by events in Brussels today and even more depressed for the future", while tweeting an article from the Telegraph highlighting: "Next time you hear the Prime Minister say that Britain must remain in the EU for the sake of our security please think of Brussels".
However, David Cameron criticised the party's comments, saying it was "not appropriate" to be drawing a link between terror attacks and immigration on such a day.
Farage doubled down, insisting he was "wholly justified" to link the Brussels attacks and EU migration rules.
He added that those campaigning for the UK to stay in the EU had already "politicised" the security issue after the Paris attacks.
The attacks have played into the hands of eurosceptic parties, particularly from the right, who are portraying imminent danger.
More than 30 people were killed in an attack in Brussels yesterday claimed by the so-called Islamic State.
Meanwhile, Marine Le Pen, leader of the National Front in France, said the attacks were the efforts of "Islamist barbarism". She added that it is important to recover controls on all national borders of the country.
"Against Islamic fundamentalism, the French require the authority of the action and the will," she added.
But French President Francois Hollande said the whole of Europe was hit by the attacks.
In Germany, Beatrix von Storch, MEP for Germany's AfD party, said on Facebook that "we have a problem in Europe. The problem is imported. And we let the problem grow everyday."
But German Chancellor Angela Merkel retorted that the attackers were "enemies of all values that Europe stands for today, and for which we as members of the European Union stand for, especially on this day, and with great pride".
Across the Atlantic, presidential hopeful Donald Trump has once again upped his anti-immigration narrative.
He said: "Time and time again I have been right about terrorism. It’s time to get tough!" while having also reiterated that the US should "close up our borders until we figure out what's going on."
"What we're doing is we're allowing thousands and thousands of these people into our country," he said on NBC. "And we're going to have nothing but problems, as sure as you're sitting there."
Yet Clinton said it is "unrealistic to say we're going to completely shut down our borders to everyone. That would stop commerce, for example, and that’s not in anybody’s interest."
She added in a speech after her victory in Arizona: "We live in a complex and dangerous world and we need a commander-in-chief that can provide leadership that is strong, smart and above all steady in taking on these threats."
The remarks from Trump, Le Pen and Farage all follow on in a similar vein to those made back in November in the wake of the Paris attacks.