A major evacuation was launched at the European Parliament after a man dressed in camouflage was arrested, and a chainsaw and gun found in his car.
Prosecutors in Brussels said the man, from Slovakia, told police officers at the scene that he wanted to meet the European president.
The incident sparked a security scare, including the evacuation of a number of offices nearby. A judge will decide tomorrow whether he faces charges.
It comes as Belgium was put on high alert after security forces foiled a plot by Islamist radicals to attack police stations across the country. Extra soldiers have been deployed.
Burned out and defaced vehicles and rubbish-strewn streets have been left in central Brussels after violent clashes erupted as more than 100,000 protesters marched against austerity measures.
Whether it is a Yes or No vote, the referendum result will be followed closely around the world, especially regarding the possible "domino effect" in Europe.
Scotland's decision is of particular interest to Flemish nationalists in Belgium and the Catalans in Spain.
ITV News Europe Editor James Mates reports from Brussels:
A pair of underpants belonging to the Mayor of Brussels has been stolen from a museum showcasing underwear belonging to Belgian politicians and artists.
Until recently one of the key exhibits at the Brussels Underpants Museum was a pair of briefs donated and signed by the Mayor of Brussels, Yvan Mayeur.
Landlord Jan Bucquoy told the website brusselnieuws.be: "We only noticed the theft last Sunday. The underpants were exhibited in the pub. It's a lot easier to steal the pants there than in the museum on the first floor".
Mr Bucquoy now intends to ask Mr Mayeur for a replacement pair.
An armed gang dressed as police officers are thought to be behind a diamond heist at Brussels Airport.
ITV News' Emma Murphy reports:
A spokesman for Brussels Airport has given details of a robbery in which an armed gang targeted a plane preparing for take-off, stealing a haul of diamonds worth millions of pounds.
An armed gang dressed as police officers are thought to be behind a diamond heist at Brussels Airport. The masked men stole a cache of stones and sped off into the darkness on Monday night, said Anja Bijnens, spokeswoman for the Brussels prosecutors' office.
It is understood the robbers targeted a security van parked near a Swiss plane. Police later found a burnt-out vehicle close to the airport. There have been conflicting reports about the value of the stolen gems, ranging from £32.3m to £300m.
An airport spokesman said the robbers made a hole in the fence and drove up to the passenger plane, which was ready to leave.
The thieves got out of the car, flashed their weapons and took the loot from the hold, said airport spokesman Jan Van Der Crujsse. Without firing a shot they drove off through the same hole in the fence, completing the theft within minutes, he added.
An armed gang have stolen a haul of diamonds during a robbery at Brussels Airport.
Belgian state broadcaster VRT is reporting that two vehicles, carrying four armed men, drove up to a security van parked near a Swiss passenger plane.
There have been conflicting reports of the value of the diamonds, but they are thought to be worth upwards of £30m.
"The men were armed and masked. There were no shots fired and nobody was injured," a spokeswoman for the Brussels prosecutor said.
Both vehicles carrying the thieves sped off after the robbery and one of them was later found, said officials, who would not comment on what was taken.
The robbers managed to get onto the tarmac after breaking through the fence that surrounds the airport, a spokesman for Brussels Airport said.
Almost 25,000 people have signed an e-petition against the installation of a 25-metre high sculpture of a Christmas tree rather than the traditional pine, in the main square of Brussels.
The sculpture, designed by French architects Pier Schneider and Francois Wunschel, has not been received well by locals with the online petition demanding city leaders, "respect for our values and our traditions" and replace the installation with a 'real' Christmas tree.