Blasts rocked the Brussels airport and a metro station in the Belgian capital, killing at least 13 people and injuring many others, police have said.
Eurostar has cancelled trains running to and from Brussels. "No trains are currently running to or from Brussels Midi," the rail service said on its Twitter feed.
Ian McCafferty, witness at Maelbeek metro station, tells Al Jazeera: "People started running when they saw smoke coming. The point of these attacks is to make you live in fear but I refuse to."
Dutch Prime Minister Mark Rutte says Belgium has "again been hit by cowardly and murderous attacks. Our hearts go out to the victims and next of kin. The Netherlands stands ready to help and support our southern neighbours in any possible way."
Eye witness at Zaventem airport tells APTN: "We were having lunch on the fourth floor and then suddenly heard a loud first explosion ... Then, after a few seconds, there was a second large explosion; many injured were lying near the checkout desk on the ground."
Belgium's federal prosecutor says the deadly explosions at Brussels airport were carried out by a suicide bomber, broadcasters VTM and RTBF report.
French President Francois Hollande calls emergency meeting of senior government ministers after the Brussels blasts.
Ten people killed after explosion in Brussels metro, Belgian public broadcaster VRT reports.
European Council President Donald Tusk says he is "appalled by the bombings this morning at Zavantem airport and the European district in Brussels which have cost several innocent lives and injured many others".
Tom, an intern working at the Brussels airport, tells Al Jazeera: "When I heard the first explosion, a lot of people started screaming and running. When I heard the second explosion, which was about 30 seconds after the first one, everything got chaotic; I could see panic on everyone's face, blood on their bodies."
French authorities decide to reinforce security at airports, train stations and metros after the attacks in Brussels.
Belgian federal prosecutor Frederic Van Leeuw calls the blasts in Brussels "terrorist attacks".
The death toll at Maelbeek metro station in Brussels is at 15, Belgian media say
Facebook activates its "safety check" system to help people check on friends and loved ones following the attacks in Brussels.
Speaking from Paris, French President Francois Hollande has just told reporters that "terrorists struck Brussels, but it was Europe that was targeted".
Peter Altmaier, the chief of staff of German Chancellor Angela Merkel, has tweeted "terrorists will never win" as he called for solidarity with Belgium.
Over 200 flights to Brussels have been diverted or cancelled after Tuesday's explosions, according to the flight tracking service Flightradar24.