Bombs struck the Brussels airport and one of the city's metro stations Tuesday, killing at least 31 people and wounding dozens, as a European capital was again locked down amid heightened security threats.

The two airport blasts, at least one of which was blamed on a suicide bomber, left behind a chaotic scene of splattered blood in the departure lounge as windows were blown out, ceilings collapsed and travelers streamed out of the smoky building.

About an hour later, another bomb exploded on a rush-hour subway train near the European Union headquarters. Terrified passengers had to evacuate through darkened tunnels to safety.

"What we feared has happened," Belgian Prime Minister Charles Michel told reporters. "In this time of tragedy, this black moment for our country, I appeal to everyone to remain calm but also to show solidarity."

Belgium raised its terror alert to the highest level, diverting planes and trains and ordering people to stay where they were. Airports across Europe immediately tightened security.

"We are at war," French Prime Minister Manuel Valls said after a crisis meeting called by the French president. "We have been subjected for the last few months in Europe to acts of war."

European security officials have been bracing for a major attack for weeks, and warned that the Islamic State group was actively preparing to strike. The arrest Friday of a key suspect in the November attacks in Paris heightened those fears, as investigators said many more people were involved than originally thought, and that some are still on the loose.

There was no immediate claim of responsibility for Tuesday's attacks, and Michel said there was no immediate evidence linking key Paris suspect Salah Abdeslam to them. After his arrest Friday, Abdeslam told authorities he had created a new network and was planning new attacks.

At Brussels' Zaventem airport, the two explosions hit the departures area during the busy morning rush. Belgian Health Minister Maggie de Block told Belgian media that 11 people were killed and 81 injured.

Zach Mouzoun, who arrived on a flight from Geneva about 10 minutes before the first blast, told BFM television that the second, louder explosion brought down ceilings and ruptured pipes, mixing water with victims' blood.

"It was atrocious. The ceilings collapsed," he said. "There was blood everywhere, injured people, bags everywhere."

"We were walking in the debris. It was a war scene," he said.

Anthony Deloos, an airport worker for Swissport, which handles check-in and baggage services, said the first explosion took place near the Swissport counters where customers pay for overweight baggage. He and a colleague said the second blast hit near the Starbucks cafe.

"We heard a big explosion. It's like when you're in a party and suddenly your hearing goes out, from like a big noise," Deloos said, adding that shredded paper floated through the air as a colleague told him to run.

"I jumped into a luggage chute to be safe," he said.

The bomb that went off an hour later on the subway train killed 20 people and injured more than 100, Brussels Mayor Yvan Majeur said.

"The metro was leaving Maelbeek station for Schuman when there was a really loud explosion," said Alexandre Brans, 32, wiping blood from his face. "It was panic everywhere. There were a lot of people in the metro."

Near the entrance to the station, rescue workers set up a makeshift medical treatment center in a pub. Dazed and shocked morning commuters streamed from the metro entrances as police tried to set up a security cordon.

The metro shut down after the attacks, as did the airport. More than 200 flights to Brussels were diverted or canceled, according to flight tracking service Flightradar24.

At least one and possibly two Kalashnikovs were found in the departure lounge at the airport, according to a European security official in contact with Belgian police who spoke on condition of anonymity because he was not authorized to speak about the ongoing investigation.

It was not immediately clear whether the firearms were used in the attacks.

Amateur video showed passengers fleeing as quickly as they could.

In a video shown on France's i-Tele television, passengers, including a child running with a backpack, dashed out of the terminal in different directions as they tugged luggage. Another image showed a security officer patrolling inside a hall with blown-out paneling and what appeared to be ceiling insulation covering the floor.

Marc Noel, 63, was about to board a Delta flight to Atlanta, to return to his home in Raleigh, North Carolina. A Belgian native, Noel says he was in an airport shop buying automobile magazines when the first explosion occurred 50 yards away.

"People were crying, shouting, children. It was a horrible experience," he told AP. He said his decision to shop might have saved his life. "I would probably have been in that place when the bomb went off."