The BBC's Anna Holligan tweets:

The Brussels attacks targeted the city's main airport and a metro station - areas with little or no security.

Security officials say such attacks - like those in Paris last year and in London in 2005 - are almost impossible to prevent.

The BBC's transport correspondent Richard Westcott looks at what the Brussels attacks could mean for travellers.

Image source, AFP

If you are just joining us, here is a round-up of the latest developments from Brussels:

• Police have issued a wanted notice for a man in a hat who was caught on CCTV pushing a laden luggage trolley at Zaventem airport.
• An explosive device containing nails, chemical products and a flag of the so-called Islamic State were found during raids in Brussels.
• Police operations were under way at several points in the city but a lockdown imposed after the attacks has been eased.
• The threat level across Belgium has been raised to maximum, and troops have been deployed in Brussels.
• Security has been beefed up in many other European cities.
• More than 30 people are believed to have been killed in the attacks.
• Here is our comprehensive round-up of all the days events.
• We have also talked to some witnesses of the attacks.
• And here are some of the most striking pictures of the day.

The attacks in Brussels happened just four days after Salah Abdeslam, a key suspect in last year's Paris attacks, was arrested in the Belgian capital.  

His lawyer says he is co-operating with the authorities - prompting suggestions that Tuesday's attacks were carried out either in revenge for Abdeslam's arrest, or by members of a cell anxious to act before they too were caught. 

The BBC security correspondent Frank Gardner examines the possible links between the Paris and Brussels attacks and why Belgium appears to be at the centre of European jihadism.  

Image source, Reuters

In a sign of solidarity with Belgium, the Eiffel Tower in Paris has been lit in black, yellow and red - the colours of the Belgian national flag

Image source, AFP

In a televised address to the nation, King Philippe of Belgium said he and Queen Mathilde "share the pain" of all those who had suffered in the attacks.

"Today our country is in mourning. For each of us this 22 March will never be a day like any other. The broken lives. The profound injuries. This suffering is that of the whole country," he said.

"We express all our support to the members of the emergency and security services. And to all those who spontaneously offered aid" he added. 

"Faced by the threat we will continue to respond together."

Katya Adler
Europe Editor

Image source, Reuters

In Belgium, on the streets and amongst political and security chiefs Tuesday's attacks are seen not as an isolated incident but as part of a far bigger picture - an attack on Europe - in the heart of Europe. 

Zaventem airport is a hub for cross continental flight. The metro bombing happened minutes away from the European Parliament. 

The bombings took place four days after a key suspect in the Paris attacks was arrested here. 

The bombers presumed message today is "no-one is safe, whoever they are, wherever they are".   

And Europe reacted immediately - with many capitals raising alert levels and boosting security at their main airports. 

But terrorism is only one of a number of dangers to the EU. 

The migration crisis - all but forgotten today - has driven EU countries apart, frightened their populations and questioned the EU's ability and credibility. 

There is the euro crisis too - ever present, always lowering with whispers of "Grexit", Greece being forced out of the currency never far away - and finally "Brexit" - the very real possibility that the UK, one of the EU's most prominent members could leave the club. 

And the fear, too, that others might follow.

More on presser with federal prosecutor - Frederic Van Leeuw said searches in Schaerbeek area of Brussels led to discovery of explosive device containing nails, chemical products and an Islamic State flag.

BBC News Europe producer Piers Scholfield tweets:

US journalist Shannon Butler at Orlando Sanford airport in Florida says a plane that arrived from Brussels was "met by cops after landing".

She adds that security is "tight" at the airport.

Belgian Federal Prosecutor Frederic van Leeuw has said there are still no confirmed figures regarding fatalities and casualties; various controlled explosions have taken place during the day on suspect packages by the bomb squad and it is too early to make a direct link with last year's Paris attacks.

He also said there were ongoing investigations around the country.

Belgian Prime Minister Charles Michel has just held another press conference, and these are the main points arising from it:

• National Security Council met tonight
• New assessment expected tomorrow
• Crisis Centre and Security Services have adopted additional measures
• Everything has been put in place to re-open public transport
• There will be extra security personnel on public transport
• There will be three days of national mourning
• Cabinet will meet on Wednesday morning – extraordinary meeting

The BBC's Matthew Price in Brussels says crowds are now gathering outside the former bourse - stock exchange -  in the Belgian capital. 

Belgium's CrisisCenter is now advising passengers to delay their train journeys, warning there are long queues at the city's railway stations.

Police are also checking passengers at the main terminals.

Image source, Reuters

The BBC's Stephen Sackur in Brussels says the message from the Belgian authorities to city residents now is "begin to go about your business, but be very cautious".

Image source, Getty Images

Police have issued a wanted notice for a man suspected of involvement in the bomb attacks at Brussels airport. 

Belgian media earlier carried pictures from airport security cameras showing two people in black and a third person wearing a hat and following just behind.  

There is at least 10km (six miles) of queuing traffic at the Belgian border with the Netherlands as what looks like police spot checks are carried out, the BBC's Jenny Hill reports. 

Image source, EPA

BBC Monitoring

Islamic State (IS) militants have issued an official statement claiming the Brussels attacks, following up on an earlier report from the IS-affiliated A'maq news agency claiming responsibility on behalf of the group. 

The branded statement, issued via IS's official social media accounts, warned of worse to come for "Crusader states allied against the Islamic State". 

The statement said targets in the Belgian capital had been "carefully selected", suggesting the attacks might have been centrally directed. The statement did not indicate how many IS operatives were involved. It claimed more than 40 people had been killed and over 210 injured.

BBC Trending takes a look at misleading videos circulating online purporting to show the Brussels attacks. But they are old, from an airport attack in Russia and a metro station bombing in Belarus.