World leaders responded with condemnations to Tuesday morning's terror attacks in Brussels killing at least 34 people, injuring dozen others.
France raised its terror threat to the highest level and deployed extra police to key locations in response to events in the Belgian capital.
Following an emergency meeting with President Francois Hollande, Prime Minister Manuel Valls said Europe was in a "state of war".
His interior minister, Bernard Cazeneuve, said 1,600 extra security forces would be deployed to French airports, train stations and seaports, with 400 in Paris alone.
United Nations chief Ban Ki-moon condemned Tuesday's deadly attacks in Brussels, calling them "despicable" and demanding those responsible face justice.
"The despicable attacks today struck at the heart of Belgium and the center of the European Union," said a statement from his office. "The secretary-general hopes those responsible will be swiftly brought to justice."
In Britain, Prime Minister David Cameron said he was "shocked and concerned" by the events in Brussels and promised his country would do everything to help.
He later told U.K. broadcasters: "These are appalling and savage terrorist attacks and I've just spoken to the prime minister of Belgium to give our sympathies and our condolences to the Belgian people, and we absolutely stand with them at this very difficult time."
"These were attacks in Belgium; they could just as well be attacks in Britain or in France or Germany or elsewhere in Europe and we need to stand together against these appalling terrorists and make sure they can never win," he said.
A meeting of Cobra, the U.K. government emergency response committee, took place Tuesday morning. Major British airports, including London Heathrow and Gatwick, announced heightened security measures, including armed police patrols.
London Mayor Boris Johnson said: "I want to stress very much that we know of no immediate implications for this city but obviously we're stepping up the presence at transport hubs and major airports but that has nothing to do with any information we've received, it's purely for reassurance purposes."
Speaking in Cuba, U.S. President Barack Obama said the U.S. stood in solidarity with the Belgian people "in condemning these outrageous attacks against innocent people".
"We will do whatever is necessary to support our friend and ally Belgium in bringing to justice those who are responsible," he said. "This is yet another reminder that the world must unite, we must be together regardless of nationality, or race or faith in fighting against the scourge of terrorism. We can and we will defeat those who threaten the safety and security of people all around the world."
Obama spoke by phone with Belgian Prime Minister Charles Michel to offer his condolences directly, and extend U.S. support, according to the White House.
During the call, Obama "reaffirmed the steadfast support of the United States for Belgium, and offered any assistance necessary in investigating these attacks and bringing those responsible to justice".
In Washington, Israeli Prime Minister Benjamin Netanyahu expressed his condolences for those killed in the attacks.
"The chain of attacks from Paris to San Bernardino to Istanbul to the Ivory Coast and now to Brussels, and the daily attacks in Israel, this is one continuous assault on all of us," he said while addressing the nation's largest pro-Israel gathering via video link.
Shares in airlines and travel companies dropped sharply as stock exchanges opened on Tuesday. IAG, the parent company of British Airways and Iberia, and easyJet were both down by around 4 percent in the first half hour of trading.
International rail services to Brussels from Paris and London were also canceled.
Spain's foreign minister, Jose Manuel Garcia-Margallo, blamed the attack on Daesh, calling it "a kind of terrorist cancer that is extending all over the world." He confirmed Spain's terrorist threat level would remain at Level 4, the second highest stage.
President Recep Tayyip Erdoğan on Tuesday strongly condemned the terrorist attacks in Brussels and said that Turkey stands in solidarity with Belgium.
According to a written statement on the Presidency's website, Erdoğan said that the people of Turkey, who have faced similar ruthless attacks by terrorist organizations, deeply feel the pain of Belgians.
"I strongly condemn the inhumane terrorist attacks carried out at Brussels' Zaventem Airport and Maelbeek subway stations" Erdoğan said and added that Turkey is ready to provide all types of assistance to its ally Belgium.
Serbian Prime Minister Aleksandar Vucic sent a telegram of condolences to the Belgian prime minister Charles Michel.
Vucic said: "The world is shocked by the terrorist attacks, which threaten all civilized values we strongly advocate. So once again I want to emphasize the importance of joint efforts that we have to put in a determined fight against terrorism."