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More than two dozen people are dead and scores are injured after explosions at Brussels' international airport and a metro station on Tuesday. 

The Belgian government has raised the terror threat to the highest level and the Belgian Crisis Centre has told the population: "Stay where you are."

"What we feared, has happened," Belgian Prime Minister Charles Michel said in a televised press conference, calling it a "black day" for the country. A Belgian prosecutor says at least one of the explosions was a suicide bombing, Agence France-Presse reports. 

The blasts come days after Paris attacks suspect Salah Abdeslam was arrested in the Molenbeek district of the Belgian capital.  

The BBC reports:

No group has said it was behind the attacks but Belgium's Interior Minister Jan Jambon said on Monday that there was a threat from revenge attacks after the capture of Salah Abdeslam.

The Belgian-born French national is said to be co-operating with police and is fighting extradition to France.

Mr. Jambon told Belgian radio: "We know that stopping one cell can... push others into action. We are aware of it in this case."

As they did after the attacks in Paris, some observers urged a measured response. 

"The purpose of terrorism is not to destroy or kill," journalist and author Simon Jenkins wrote on Tuesday morning. "It is to pursue a political cause through the massive publicity that is attached to terrifying incidents...The explosive force derives from our reaction to it, from the public attention awarded to it and from the response of the political community. Publicity and response are the terrorists' 'useful idiocies'."

He continued:

[T]he intention of the terrorist is clearly to shut down western society, to show liberal democracy to be a sham and to invoke the persecution of Muslims. Yet that is the invariable response of the security industry to these incidents. Convinced of its potency, it dare not admit there are some things against which it cannot protect us. So when incidents occur it jerks the knee and demands ever more money and ever more power. It must not be given them.

According to news outlets, Republican presidential frontrunner Donald Trump reacted by telling Fox News he would "close up borders."