The European Union’s political heart was on high alert after an explosion at metro stations near the 28-member bloc’s main buildings in Brussels.
Here is a timeline of the key events since the Paris terrorist attacks in November last year:
• November 13 - A series of explosions rock Paris in the worst terrorist assault on Europe in a decade. A number of restaurants, the Bataclan concert hall and the Stade de France are targeted. 130 people are killed in the attacks and hundreds more injured.
• November 18 - Belgian extremist Abdelhamid Abaaoud, 27, suspected of masterminding the deadly attacks in Paris, dies along with his female cousin in a police raid in the Paris suburb of Saint-Denis. A third body is later found and eight people are detained.
• November 20 - Belgian authorities file terror charges against a third Paris attacks suspect with two more already behind bars facing similar charges.
• November 21 - Brussels is placed in lockdown, remaining at maximum alert over information about an “imminent threat”, possibly a series of co-ordinated attacks at different locations.
• November 22 - Some 19 raids are carried out in Molenbeek, home to many of the Paris attackers, and other boroughs of Brussels, and there are three raids in Charleroi. A total of 16 people are arrested.
• November 23 - An explosive vest containing bolts and the same type of explosive used in the Paris attacks is found by a street cleaner in a pile of rubble in Chatillon-Montrouge, just south of Paris. A fourth suspect - one of the 16 arrested the previous day - is charged with terrorism offences; the other 15 are released.
• November 25 - French parliamentarians overwhelmingly vote to continue air strikes against Islamic State (IS) in Syria beyond early January. Schools and some subways reopen for the first time since emergency measures were imposed in the Belgian capital.
• November 26 - Belgian authorities raid three more places outside of Brussels which they say are linked to the Paris attacks, but make no arrests.
The Belgian terror threat is lowered to the second-highest level in the capital Brussels, with officials calling a threat “possible and likely”.
• December 3 - Two more suspects are arrested. A French national, identified only as Samir Z, is detained at Brussels national airport as he bids to fly to Morocco. A second man, identified as Pierre N, is arrested during a raid only hours after the detention at the airport.
• December 10 - Prosecutors find Paris suspect Salah Abdeslam’s fingerprint during a search of an apartment in Schaerbeek, Brussels. They believe it served as a bomb factory for the Paris attacks. Three handmade belts which could have been intended to transport explosives, as well as bomb-making equipment, traces of TATP explosive residue and a fingerprint from Abdeslam were found there, the Federal Prosecutor’s Office says.
• December 20 - Belgian authorities search a house in Molenbeek, and detain an unidentified person for questioning.
• December 24 - Belgian authorities arrest a ninth suspect linked to the Paris attacks, a Belgian citizen born in 1985 they say had been in contact with the suspected ringleader’s cousin.
• December 31 - A 10th suspect is arrested in the Paris terror attacks probe and numerous mobile phones seized, the Belgian prosecutor says.
• January 13 - Belgian investigators identify three homes used by suspects to prepare for the Paris attacks.
• March 15 - An Algerian man living illegally in Belgium, Mohamed Belkaid, is killed by police during a raid on an apartment building in the quiet Brussels suburb of Forest, where authorities also find a stock of ammunition and an Islamic State flag.
• March 18 - Salah Abdeslam is wounded in the leg and captured by Belgian police in a raid in the Molenbeek district of Brussels after four months on the run. Two other men are also arrested.
• March 22 - Just days after Abdeslam is captured, Brussels is hit by attacks on Zaventem airport and the city’s metro system, near a station close to European Union buildings.