Authorities in Europe 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. At least 28 are reported dead, and dozens more injured.
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.
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.
Facebook has activated its "safety check" system to help people check on friends and loved ones in the aftermath of the attacks in Brussels.
The company says Tuesday the system was put in use within hours of the three explosions at the Brussels airport and a metro station. It says the system can provide an easy way for people to mark themselves as "safe" after a major disaster or crisis so that people searching for them will know they are unharmed.
The system has been used recently to help people communicate after major floods and earthquakes as well as terrorist attacks.
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.
Gatwick airport said that "as a result of the terrible incidents in Brussels we have increased our security presence and patrols around the airport." 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.
Deadly attacks Tuesday at the Brussels airport and a metro station in the city are the latest in a string of attacks in Europe in recent years. Here are some of the most recent major ones:
— Nov. 13, 2015: Islamic State-linked extremists attack the Bataclan concert hall and other sites across Paris, killing 130 people. A key suspect in the attack, 26-year-old Salah Abdeslam, is arrested in Brussels on March 18, 2016.
— Jan. 7, 2015: A gun assault on the Paris offices of the satirical magazine Charlie Hebdo kills 12 people. Al-Qaida in the Arabian Peninsula claims responsibility for the attack, saying it was in revenge for Charlie Hebdo's depictions of the Prophet Muhammad.
— May 24, 2014: Four people are killed at the Jewish Museum in Brussels by an intruder with a Kalashnikov. The accused is a former French fighter linked to the Islamic State group in Syria.
— May 22, 2013: Two al-Qaida-inspired extremists run down British soldier Lee Rigby in a London street, then stab and hack him to death.
— March 2012: A gunman claiming links to al-Qaida kills three Jewish schoolchildren, a rabbi and three paratroopers in Toulouse, southern France.
— July 22, 2011: Anti-Muslim extremist Anders Behring Breivik plants a bomb in Oslo then launches a shooting massacre on a youth camp on Norway's Utoya island, killing 77 people, many of them teenagers.
— Nov. 2, 2011: The offices of Charlie Hebdo in Paris are firebombed after the satirical magazine runs a cover featuring a caricature of the Prophet Muhammad. No one is injured.
— July 7, 2005: 52 commuters are killed in London when four al Qaida-inspired suicide bombers blow themselves up on three subway trains and a bus.
— March 11, 2004: Bombs on three commuter trains in rush-hour, bound to Madrid's Atocha station, kill 191 people in Europe's worst Islamic extremist attack.
Associated Press writers Jill Lawless and Danica Kirka in London, Frank Jordans in Berlin, Nataliya Vasilyeva in Moscow, Ciaran Giles in Madrid, Alison Mutler in Bucharest, Monika Scislowska in Warsaw, Karel Janicek in Prague, George Jahn in Vienna and Derek Gatopoulos in Athens contributed to this report.