European countries have raised security levels following the attacks on an airport and metro station in Brussels.
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Belgium, France, the Netherlands, Britain, Denmark, Sweden and Finland have beefed up patrol presence at borders and airports.
Belgium's Interior Ministry said the country had raised its security alert to the maximum level four across the country. The military is sending 225 extra troops to Brussels, Belgian news agency Belga said.
British airports have increased security and Prime Minister David Cameron said he would convene the government's emergency committee.
Britain's official terrorist threat level stands at "severe," the second-highest level on a five-point scale, meaning an attack is highly likely.
Gatwick airport said that "as a result of the terrible incidents in Brussels we have increased our security presence and patrols around the airport." Heathrow said it was working with police to provide a "high-visibility" presence on light of the attacks.
French President Francois Hollande called an emergency meeting of senior government ministers on Tuesday after the explosions.
Prime Minister Manuel Valls, Interior Minister Bernard Cazeneuve and Defense Minister Jean-Yves Le Drian were among those present, according to the president's office.
Meanwhile, the French interior minister said he has decided to deploy 1,600 additional police officers to man the country's borders and transport.
Dutch police stepped up security patrols at airports and tightened checks at borders after the attacks, the country's security agency said.
Travellers passing through Amsterdam's Schiphol Airport reported delays and a heavy police presence. Security agencies declined to give details of any further measures taken, but maintained the national threat level at "substantial", one notch below the highest.
Flights were diverted from Brussels to Amsterdam following attacks at the Belgian capital's airport. Trains heading south to Belgium were subject to indefinite delays, Dutch state railways said.
Police in Denmark, Sweden and Finland have stepped up security at airports and public places.
Danish police said they had increased patrols at Copenhagen airport and other key points in the city.
"We are aware of what has happened in Brussels. Therefore you will see more police in the airport and at key points in Copenhagen," Danish police said on its official Twitter page.
Danish authorities have been on high alert since two people were killed in shooting attacks on a free speech event and a synagogue in Copenhagen in February last year.
Police in Sweden said they had reinforced their presence at airports and taken increased security measures at other public places.
Finnish Interior Minister Petteri Orpo said on Twitter "Finnish security officials have increased monitoring at Helsinki-Vantaa airport".
German authorities stepped up security measures at airports, train stations and the borders with Belgium, France, the Netherlands and Luxembourg, a spokesman for the federal police said, following the attacks.
An additional police presence was noticeable at Frankfurt airport and train station on Tuesday morning, a Reuters eyewitness said.
Germany's Justice Minister Heiko Maas called Tuesday a "black day for Europe," vowing to be "steadfastly at the Belgians' side." He added on Twitter that "the horrible events in Brussels affect us all."
The deadly explosions at Brussels airport on Tuesday morning were carried out by two suicide bombers, Belgian authorities said. A further blast struck a metro station in the capital shortly afterward.
The blasts at the airport and metro station occurred four days after the arrest in Brussels of a suspected participant in the November militant attacks in Paris that killed 130 people.
Police had been on alert for any reprisal action in both capitals, which lie about 315 kilometers apart across an open border.