Armed police have arrested two men and already have CCTV of one of the Brussels airport bombers including the moment he detonated his suicide belt.
Every space in the city's airport is covered by four CCTV cameras, including the arrivals hall where scores of people were killed and many more were seriously injured.
Maelbeek station's surveillance network is also being used to identify the terrorists who detonated explosives there.
The Belgian Foreign Ministry has confirmed they believe some of the terrorists involved are "still at large".
A US official said security officials believed at least one suitcase bomb was detonated at Brussels Airport.
The official, who wasn't authorised to speak publicly on the early investigations, confirmed a statement by a Brussels official that there is also concrete evidence of one suicide bombing at the airport as well.
Two suspects have been arrested at gunpoint at Brussels North railway a mile from Maelbeek subway. It is not yet known if they were linked to the bombings.
A third suspect has been arrested on a train near Amsterdam and a suspect package at Gard du Nord in Paris delayed Eurostar services.
Police are already looking at hours of CCTV footage before and after the explosions as they hunt for those behind the attacks.
But security sources said they believed they had video footage of one of the terrorists moments before he detonated one of three bombs.
At least two people in Brussels have been arrested outside the city's North railway station, about two kilometres from the Maelbeek subway.
There were also reports the Tihange nuclear power plant, around 90km from the capital, is being evacuated of all non-essential staff as Belgium raised security to its maximum level.
Police and special forces are looking for known members of any terror cell who may be planning more attacks.
They will also round up anyone who may pose a threat to the public, or acting suspicously, in an attempt to foil any more attacks.
Intelligence agencies have been on alert for possible attacks since the arrest in Belgium of accused Paris attacks conspirator Salah Abdeslam. It is unclear if the Brussels bombings were already planned and set in motion by his or another existing network, or if they were a direct response to Abdeslam's arrest.
The explosives used in the Brussels airport attack appeared sophisticated, and investigators will examine them to see if they bear the same characteristics as those used in Paris last year.
Special forces are patrolling the streets of the Belgian capital in case of more bombings or marauding gunmen.
Armed police have been deployed to airports, train stations, ports and border crossings around the world.
From New York to Moscow, security measures were stepped up at transport hubs across the globe with police and military personnel carrying out extra high-visibility patrols and additional checks.
The border between France and Belgium was closed, Eurostar services in and out of Brussels were suspended and airlines to and from other destinations were warning passengers of potential disruption as the effects of this morning's terror attacks were felt around the world.
Train and bus stations, ports and road checkpoints were also put on high alert, with armed police on the streets in major global cities including London and New York, sniffer dogs deployed and extensive border checks put in place.
Police and soldiers carrying guns were also seen at airports in the Netherlands, Germany, and Italy, while in France - still shaken from the November attacks in Paris - officers remained on high alert.
Troops have also been deployed other key locations across Brussels as Belgium raised its security level to "maximum" in the wake of the attacks.
The entire border between Belgium and France was placed on lock-down, and the Thalys train service - which travels between France, Belgium and the Netherlands - was halted in the wake of the explosions, the operator said.
- Daily Mail