Authorities in Europe and beyond have tightened security at airports, on subways, at the borders and on city streets after deadly attacks Tuesday on the Brussels airport and its subway system.

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 plainclothes officers at key points across the bloc.

More than 200 flights to Brussels were diverted or canceled, according to flight tracking service Flightradar24.

The Paris airport authority said security was tightened at all Paris airports soon after the Brussels explosions on Tuesday morning. Airports in London, Prague, Amsterdam, Vienna, and many others, also saw increased security. The attacks come just days after the main suspect in the Nov. 13 Paris attacks was arrested in Brussels on Friday.

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 toughest across Europe. There have been mandatory checks at the entrances to airports since a 2011 suicide bombing at Moscow's Domodedovo airport that killed 37.

Egypt also said it was increasing security, with top security officials asked to personally handle security checks inside airports and in outside areas like hotels and car parks.

Egypt has been working to improve its security after a Russian jet was brought down last October by extremists last after taking off from Sharm el-Sheikh International Airport, killing all 224 people on board. Moscow said it was brought down by an explosive device, and a local branch of the extremist Islamic State group has claimed responsibility for planting it.

London's Gatwick airport said that "as a result of the terrible incidents in Brussels we have increased our security presence and patrols around the airport." Nearby Heathrow said it was working with police to provide a "high-visibility" presence in light of the attacks.

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

The British, Dutch and Polish governments convened emergency meetings as they beefed up security at airports.

Dutch Prime Minister Mark Rutte and Britain's David Cameron vowed to help Belgium.

"Our thoughts are there, in Brussels, and we are praying for the victims," said Polish Prime Minister Beata Szydlo, who canceled a routine news conference to attend an emergency meeting with her government security council.

Austrian Interior Ministry spokesman Karl-Heinz Grundboeck said more police are on the streets and at airports in Vienna and other major Austrian cities even though there appears to be no "Austria connection."

Spain's Interior Ministry said officials would meet later Tuesday to discuss the situation but that for the moment Spain was maintaining its Security Alert Level 4 — one step below the maximum — that has been in place since 2015 extremist attacks in France, Kuwait and Tunisia.

In Greece, police added additional security at airports, metro stations and embassies with uniformed and plain-clothed officers.

But Greece's government spokeswoman Olga Gerovasili said there were no additional security measures being taken for refugees and migrants following the Brussels attacks.

"We are not making any linkage between those two issues. That would be a defeat for Europe," Gerovasili said.