Key installations on alert across Europe

With Brussels on lockdown and the French prime minister saying that Europe is "at war," European leaders held emergency security meetings and deployed more police, explosives experts, sniffer dogs and officers in plain clothes at key points across the bloc.

Airports in London, Prague, Amsterdam, Vienna and many others have imposed increased security. At London's main airport, Heathrow, police were "providing a high visibility presence." Meanwhile, extra police are being deployed in London and other cities across the EU..

Dutch military police are carrying out extra patrols at Schiphol Airport in Amsterdam. The increased security comes in the wake of the explosions in Brussels, which killed many innocent bystanders and injured nearly 200 others.

Gatwick airport in the UK said that as a result of the attacks in Brussels, it has increased its security presence and patrols around the airport. London-Heathrow said it was working with police to provide a "high-visibility" presence in light of the attacks.

German authorities have also stepped up security measures at airports, train stations and the borders with Belgium, France, the Netherlands and Luxembourg, following the explosions in Brussels. The additional police presence was noticeable at Frankfurt airport - Germany's largest - and the city's train station on Tuesday morning.

German national rail system, Deutsche Bahn, has halted its high-speed service from Germany to Brussels. The company said its ICE trains are now stopping at the border city of Aachen.

The Paris airport authority said security was tightened at all Paris airports soon after the Brussels explosions on Tuesday morning. A full deployment of security officials was underway at all eight terminals of Charles de Gaulle airport and its two railway stations. Additional patrols were also dispatched to Orly airport in southern Paris and the southern city of Toulouse.

In Moscow, Russian Transport Minister Maxim Sokolov told Russian news agencies that authorities will "re-evaluate security" at Russian airports, although its measures are already among some of the tightest across Europe.