Hassan, who is also known as Afweyne or "Loud Mouth," arrived in Brussels on a plane from Kenya and was promptly arrested on piracy and hostage taking charges, Flanders News, Brussels, reported.
A high-ranking Somali official who was traveling with Hassan was also arrested.
It was unclear what the two men were doing in Brussels, the report said.
Hassan is accused of attacking a number of ships off the Somali coast and making hundreds of millions of dollars as a result of piracy. He is also know for smuggling the drug qat.
Last year, Hassan announced that he was giving up piracy to enter the world of politics.