An internal line-up clamp for welding pipe lengths end-to-end includes a clamp ring made up of a plurality of clamp ring elements that are angularly distributed around the circumference of the clamp ring. The clamp ring elements are divided into mutually opposed groups that are interdigitated such that the clamp ring elements alternate from one group to the other around the circumference of the clamp ring. A longitudinally-acting actuator system drives relative longitudinal movement of the groups to effect clamping expansion of the clamp ring by wedging interaction between the circumferentially-alternating clamp ring elements of the groups. By expanding the clamp ring in alignment with a weld plane between the abutting ends, the same clamp ring may be used to align the pipe lengths and to back a weld between them.