Brussels is on high alert for Belgian national day celebrations tomorrow, and remains tense following Islamic State-claimed suicide bomb attacks at the airport and in the metro on March 22 that left 32 people dead.
Police backed by bomb disposal teams cordoned off part of central Brussels where they surrounded the suspect individual, who aroused suspicion because he had on a long winter jacket on a hot day.
"He was studying waves and radiation. He has just been detained and police will begin questioning him now," Brussels police spokeswoman Ilse Van de Keere told AFP.
Local media footage showed troops, police and firemen in and around the cordoned-off area at Place de la Monnaie, with the streets largely deserted in what is normally a very busy shopping district near the Brussels opera house.
The latest apparent false alarm comes a month after a man with psychiatric problems who was carrying a fake suicide belt full of salt and biscuits triggered a major anti-terror operation at a Brussels shopping mall.
The March IS attacks caused shockwaves in a Belgium already on edge after it emerged that many of the IS jihadis involved in the November Paris onslaught which killed 130 people had grown up together in Brussels.
Belgium is the main source per head of population of jihadi recruits going from the European Union to fight with IS in Syria, causing deep concern that they will return home battle-hardened and even more radicalised.