An 8-year-old boy was injured Saturday afternoon in Gaithersburg when a slab of concrete from a wall fell on him, pinning him from the waist down, a family friend and neighbor said Sunday.

The boy and his younger brother were sitting on a stone wall in the Waterstone residential association where they live, at Father Hurley Boulevard and Crystal Rock Drive. When they jumped off the wall, which is about 4 feet high and 16 feet long, the stone came loose, falling on top of the 8-year-old, according to the family friend, Lori Sommers.

His mother and grandfather lifted the stone slab, which was about 8 feet long and 2 feet wide, off the boy, before calling an ambulance, Sommers said.

The boy is at Children’s Hospital, undergoing surgery on one of his legs, which was severely broken. The other leg also was broken, Sommers said, but did not require surgery.

According to Sommers, only sand was holding the stone in place on the wall.