Update: Authorities now believe the man was a student measuring radiation levels in the Belgian capital.
Police spokeswoman Ilse Van de Keere said no explosives were found and the man has been detained for questioning.
"Everything is under control," she added.
Earlier:A man is being held under armed police guard in central Brussels after wires were spotted hanging from his coat.
Several hours after police were alerted to the man, he remains under guard near a library adjacent to Royal Monnaie Opera.
A bomb squad is being kept on standby nearby.
The incident comes as Belgium remains on high alert after 32 people were killed in suicide blasts at Brussels Airport in March.
After the Bastille Day attack in Nice that left 84 people dead last week, security in Brussels had been further stepped up ahead of Thursday's independence day celebrations.