The suspected shooter at the al-Noor Islamic Centre near the country's capital was described as "a young white man," the police added.
The victim was a 75-year-old member of the congregation, mosque director Irfan Mushtaq told TV2.
"The man carried two shotgun-like weapons and a pistol. He broke through a glass door and fired shots," he said.The shooter, who wore body armor, was overpowered by members of the mosque before police arrived, Mushtaq added.