At least 31 people have been killed and dozens injured in attacks at Brussels international airport and a city metro station.
Twin blasts at Zaventem airport about 8am local time (8pm NZT) killed 11 people, followed by an explosion at Maelbeek metro station near the EU's headquarters an hour later that left a further 20 people dead.
Almost 200 people were injured, many of them severely.
The Islamic State (IS) group claimed responsibility for the attacks in a statement issued on the IS-linked Amaq agency.
Read how events unfolded here
Belgian prosecutor Frederic Van Leeuw said searches in Schaerbeek area of Brussels led to the discovery of an explosive device containing nails, chemical products and an Islamic State flag.
Some witnesses at the airport said after the first blast people fled, only to get caught in the second blast.
Zach Mouzoun, arriving on a flight from Geneva, told France's BFM television: "It was atrocious. The ceilings collapsed. There was blood everywhere, injured people, bags everywhere. We were walking in the debris. It was a war scene."
Zaventem airport is 11km northeast of Brussels and dealt with more than 23 million passengers last year.
The metro blast an hour later struck the middle carriage of a three-carriage train while it was moving away from the platform.
Alexandre Brans told AP: "The metro was leaving Maelbeek station when there was a really loud explosion. It was panic everywhere. There were a lot of people in the metro."
The station is close to EU institutions. The European Commission, the EU's executive arm, told employees to stay indoors or at home. All meetings at EU institutions were cancelled.
Ryan McGhee, a catering worker at a college in Brussels, told the BBC: "The entire city is in lockdown. People are calm at the moment but the atmosphere is tense."
The bombings came four days after Salah Abdeslam, the main fugitive in the Paris attacks, was seized in Brussels.
The state-owned Belga news agency reported that shots were fired and shouts in Arabic were heard before the two explosions.
Public broadcaster VRT said an assault rifle was found next to a dead attacker. Private broadcaster VTM added that an unexploded bomb belt had been found.
Reports said it would be detonated safely soon.
The Ministry of Foreign Affairs and Trade (MFAT) said it had received no indication any New Zealanders were caught up in the Brussels attacks.
It said 72 New Zealanders were registered on the Safe Travel website as being in Belgium and a message has been sent to all of them asking them to get in touch with family in New Zealand.
A spokesperson said a steady stream of positive responses has been received overnight and the New Zealand Embassy in Brussels continued to keep in close contact with the Belgian Crisis Centre.
Anyone with concerns about family members in Brussels are asked to make direct contact with them in the first instance, but can call the Ministry of Foreign Affairs and Trade on 04 439 8000.
Belgium raised its terrorism alert to its highest level. Three days of national mourning were declared in the wake of the attacks.