Thousands of armed police were deployed to European borders, transport hubs, and streets on Tuesday following triple explosions at a metro station and airport in Brussels which killed dozens of people and were later claimed by the Islamic State.

Crisis meetings were held by governments across the continent, including in Poland, the UK, France, Germany, and Holland, to discuss the threat level to neighboring countries and decide how to respond.

Troops have been deployed across Belgium, with the country's terror alert raised to its maximum level following the attacks, which targeted the departures hall at the airport in Zaventem and the Maelbeek metro station in the city center. Flights to Belgium were suspended or diverted, and Eurostar services to Brussels were stopped.

Extra security measures including armed police and sniffer dogs were also put in place at airports including in the UK, Germany, Holland, Austria, the Czech Republic, and Greece. Security was also ramped up in major US cities including New York, Washington DC, and Los Angeles.

In a rare joint statement, leaders of the 28 member states of the European Union (EU) called Tuesday's events in Brussels "an attack on our open democratic society."

"The European Union and its member states stand firm with Belgium in solidarity and are determined to face this threat together with all necessary means," it said. "This latest attack only strengthens our resolve to defend the European values and tolerance from the attacks of the intolerant. We will be united and firm in the fight against hatred, violent extremism and terrorism."

French President Francois Hollande said while Belgium may have been the target of Tuesday's attack, "the whole of Europe has been hit."

Following an emergency meeting of senior government ministers, French Prime Minister Manuel Valls said: "We are at war. Over the past few months in Europe, we have endured several acts of war."

Interior Minister Bernard Cazeneuve said 1,600 troops would be deployed at border checkpoints, and on air, sea and rail transport infrastructure. A total of 400 additional police officers will boost security in the greater Paris area.

Parisians will also have to face additional security measures, Cazeneuve added.

"Four point measures have been decided for public transportation. Firstly, entrances to public zones in transport infrastructures will, from this point forward, be reserved only to people with a ticket and/or an identification card," he said.

British Prime Minister David Cameron condemned the attacks, saying British people knew how Belgians felt as they had experienced similar attacks in London in July 2007. "We face a very real terrorist threat right across the different countries of Europe and we have to meet that with everything we have," he said.

EU foreign policy chief Federica Mogherini broke down in tears as she cut short a news conference in Jordan upon hearing news of the attacks. "It's a very sad day for Europe as Europe and its capital are suffering the same pain that this region has known every single day — plagued in Syria, plagued elsewhere," she said in the Jordanian capital Amman.

Security services have been on a high state of alert across western Europe since November's militant attacks in Paris, which killed 130 people. The Brussels blasts occurred four days after police in Brussels arrested Salah Abdeslam, the main fugitive in the Paris attacks.

Belgian police had been on alert for any reprisal action, but the attacks took place in crowded public areas where people and bags are not searched.

While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.

Although there may be discreet surveillance, there is nothing to prevent member of the public walking in to the departure hall at Brussels' Zaventem airport with heavy baggage.

Following an attempted ramraid attack at Glasgow Airport in 2007, several airports stepped up security at entrances by altering the pick-up and drop-off zones to prevent private cars getting too close to terminal buildings.