- 2 blasts at Zaventem airport, 1 at Maalbek metro station

- Terror alert raised to highest level in Belgium

- 'Possible revenge attack' after capture of Pairs attack suspect

A series of explosions ripped through Brussels airport and a metro train on Tuesday, killing around 35 people and injuring more than 200 in the latest attacks to rock Europe.

Security was tightened across the jittery continent and transport links paralysed after the bombings that Belgian Prime Minister Charles Michel branded "blind, violent and cowardly".

"This is a day of tragedy, a black day," Michel said on national television.

Foreign Minister Didier Reynders warned that authorities fear suspects could still be at large in the city that is home to both NATO and the European Union.

The bloodshed came just four days after the dramatic arrest in Brussels of Salah Abdeslam -- the prime suspect in the Paris terror attacks claimed by the Islamic State group -- after four months on the run.

Belgian authorities had been on alert after Abdeslam, Europe's most wanted man, told investigators he had been planning an attack on Brussels.

Also READ: 'Accomplice' of suspect named

Two blasts shattered the main hall of Zaventem Airport at around 8:00am (0700 GMT), with prosecutor Frederic Van Leeuw saying there was probably at least one suicide bomber.

A third hit a train at Maalbeek metro station in the heart of the city's EU quarter, just as commuters were making their way to work in rush hour.

Pierre Meys, spokesman for the Brussels fire brigade, told AFP at least 14 people had been killed at the airport, while Brussels mayor Yvan Mayeur said "around 20" died in the underground blast. More than 200 people have been wounded, several critically.

Witnesses said victims lay in pools of blood at the airport, their limbs blown off. There were chaotic scenes as passengers fled in panic, with a thick plume of smoke rising from the main terminal building.

"A man shouted a few words in Arabic and then I heard a huge blast," airport baggage security officer Alphonse Lyoura told AFP, his hands bloodied.

"A lot of people lost limbs. One man had lost both legs and there was a policeman with a totally mangled leg."

- Europe's capital in lockdown -

An army team later blew up a suspect package at the shuttered airport, with media reporting police had found an unexploded suicide vest.

At Maalbeek station, paramedics tended to commuters with bloodied faces as the streets filled with the wailing of sirens.

At least two Polish nationals and a Briton were confirmed among the injured in a city that is the EU's symbolic capital.

The bombings triggered a transport shutdown, with flights halted and metro, tram and bus services all suspended.

Airports across Europe swiftly announced they were boosting security, including in London, Paris, and Frankfurt. Across the Atlantic, New York and Washington ordered extra counter-terror officers to crowded areas and train stations.

Leaders across Europe reacted with shock and solidarity, urging closer counter-terror cooperation on a continent that has been on high alert for months.

"The whole of Europe has been hit," said French President Francois Hollande, whose country is still reeling from jihadist attacks in Paris that killed 130 people in November.

British Prime Minister David Cameron warned of the "very real" terrorist threat faced by countries across Europe, declaring: "We will never left these terrorists win."

Russia and Turkey -- also targets of deadly attacks in the last eight months -- said the blasts highlighted the need to fight terrorism of every hue and across all borders.

Brussels residents were told to stay inside. Security was also beefed up at Belgium's nuclear plants -- where non-essential staff were sent home -- and at EU buildings in the French city of Strasbourg, home to the European Parliament.

Interior Minister Jan Jambon announced that Belgium's terror threat had been raised from three to a maximum of four, and the country's national security council was due to meet.

And after rumours of arrests and searches, authorities told media to halt all reporting on the investigation into the bombings, "so as not to harm the inquiry".

- Bloody week for Brussels –

In Cairo, the head of Sunni Islam's leading seat of learning, Al-Azhar, said the attacks "violate the tolerant teachings of Islam".

Messages of solidarity poured out on social media, with thousands of people sharing images of beloved Belgian cartoon character Tintin in tears.

It has been a week of drama and bloodshed in Brussels. Last Tuesday saw a shootout in the city's south that saw a Kalashnikov-wielding man killed and four police officers wounded.

Investigators believe key Paris suspect Abdeslam slipped out of the apartment as the gunbattle broke out. He was arrested three days later in Brussels' gritty Molenbeek district -- just around the corner from his family home.

Foreign Minister Reynders said at the weekend that Abdeslam -- believed to have played a key logistical role in the Paris carnage -- had told investigators he was planning some sort of new attack.

Shiraz Maher, a radicalisation expert at Kings College London, said it was "very likely that this attack will have been planned and prepared well in advance of last week's arrest of Salah Abdeslam".

"It therefore points to the existence of a broad and sophisticated terrorist network in Belgium, that extends beyond the one which attacked France last year," Maher said.