"We are trying to stabilize the situation to assure security on other sites for which there is still concern," Michel told reporters. "This is a dark moment for our nation. We need calm and solidarity."

The synchronized attacks during the morning rush hour struck at the heart of the EU at a time when a deluge of refugees from the Middle East is testing the 28-nation bloc's dedication to open borders and stirring up anti-foreigner demagoguery. Prime Minister David Cameron criticized the U.K. Independence Party for seeking to use the assaults to make the case for Britain to leave the EU. German Chancellor Angela Merkel, who is set to speak on the assaults later Tuesday, is fighting opponents within her own government pushing her to back away from her open-door policy for asylum seekers.

Attack on Freedom

The underground bombing wreaked carnage down the street from where EU leaders on Friday struck a deal with Turkey to address the region's biggest refugee wave since World War II. The busy subway line is used by Belgian commuters, schoolchildren, tourists and bureaucrats from all over Europe -- a cross-section of the global community.

Map of Brussels Attacks

Bloomberg

"It seems clear that the attack targets -- an international airport, a subway station near EU institutions -- indicate that this terrorist attack wasn't just aimed at Belgium, but at our freedom, our freedom of movement," German Interior Minister Thomas de Maiziere told reporters in Berlin.

The timing, only four days after the arrest in Brussels of Salah Abdeslam, believed to be the only surviving perpetrator of the Paris massacres, was a brazen signal of the unrelenting threat Europe faces even with some terrorist operatives behind bars. The attack also shows the vulnerability to terror in open societies such as Belgium, where authorities have been on alert since the November slaughter in the French capital after the discovery that some of the suspects had lived in Brussels.

More than 180 injured in three explosions in Belgian capital

Brussels at standstill as authorities search for more bombs

Belgium was on the highest terror-alert level after three bombings in Brussels on Tuesday killed at least 31, injured more than 180 and raised fears of a string of follow-up attacks.

Prime Minister Charles Michel, condemning the deadliest terror attack on Belgian soil ever as "a violent and cowardly" assault, deployed Belgium's military to secure the capital after two explosions at the airport and a bombing at a subway station a short walk from the European Union's headquarters.