The federal prosecutors' office said that antiterror police picked the man up on Saturday, acting on information suggesting an attack on the embassy might be imminent. Belgian state broadcaster RTBF said the man, who denies the allegations, has been under police surveillance for some time.
It said that he was recently spotted acting suspiciously near the embassy. The U.S. embassy could not be immediately reached for confirmation or details. Brussels, where the headquarters of the European Union and the NATO military alliance are, has been a target in the past for extremist attacks.
Thirty-two people were killed and hundreds wounded on March 22, 2016, when suicide bombers blew themselves up at the city's airport and on an underground train. Several extremists involved in other attacks around the world have lived in, or transited through, Brussels.