The toy mini-figures, made by American Will Chapman, includes a masked terrorist bandit with an assault rifle, grenade launcher and belt of explosives.
Shocked by the playthings, British Muslim organisation the Ramadhan Foundation has branded the figurines "absolutely disgusting".
Chief executive Mohammed Shafiq said the figures were "glorifying terrorism".
He said: "I don't think there's any difference between someone that shouts hatred through a megaphone and someone that creates a doll that glorifies terrorists.
It belittles the pain the families of victims of terror have gone through.Ramadhan Foundation chief executive Mohammed Shafiq
"As a parent myself, I'm going to teach my children respect for the law and respect for each and every community.
"These are the lessons parents should be giving to their children - not lessons about weapons and violence."
Father-of-three Mr Chapman boasts on his website that his nine-year-old son gave him the idea for the range.
The site advertises 31 different Lego-style weapons, weapons packs and 10 miniature militant figurines.
Other fighters in the range include World War Two troops, US marines - and a Nazi SS officer.
A spokesman for Lego UK said they were "not associated" with the toys being sold by BrickArms, "which have been customised without LEGO UK's knowledge or permission."
He added Lego is "committed to developing toys which enrich childhood by encouraging imaginative and creative play - and does not endorse products that do not fit with this philosophy."