Facebook founder and CEO Mark Zuckerberg. REUTERS/Brian Snyder Facebook has activated its Safety Check feature after a series of apparent terrorist attacks in Brussels have killed more than 20 people and brought the city to a standstill.

On Tuesday morning, there were two explosions at an airport in Brussels — as well as further explosions at city Metro stations. At least 21 were reported dead, with dozens more injured.

There were immediately calls for Facebook to activate its Safety Check feature, a tool that allows people in the area to check themselves in as safe after natural disasters or terrorist attacks.

It was activated in Paris after the attacks there in November — the first time it was used in the aftermath of a non-natural disaster. Since then it has been activated after attacks in Nigeria and Ankara, Turkey.

It took more than two hours for Facebook to activate the tool in Brussels, prompting some frustration. There are multiple reports that phone lines and mobile networks are being overloaded, making it difficult for people to contact loved ones.

It's possible that Facebook waited to ensure there were not going to be further attacks before activating the feature.

You can see which of your friends have been marked safe by going here.