One man was arrested - a Muslim who said he was born in 1978 - in the attack on the mosque in Anderlecht, a suburb of Brussels, the Belgian capital, said Jean-Marc Meilleur, a spokesman for the Brussels Prosecutor's Office.
"It seemed that this person showed up and pulled out a knife and an ax, and that he spread flammable products - petrol we assume - in order to start a fire and threaten the mosque occupants," Meilleur said.
He said the man's name and age could not be verified, as he had no identification papers. He said the man had been locked in a room by worshippers at the mosque before authorities apprehended him.
Marie Verbeck, a police spokeswoman, told The Associated Press that no other people were being sought in connection with the attack.
One other person was lightly injured, Meilleur said. He said the attack took place between 6 and 7 p.m. local time.
An Associated Press photographer at the scene saw fire damage and people gathered outside the mosque, some of whom were praying.