A 17-year-old boy is seriously injured after becoming trapped by a concrete slab in Sydney's inner west.

Emergency services have freed the teen after he was trapped from the waist down at Pine St, Earlwood this morning.

The boy fell into a trench at around 10am.

A NSW Ambulance spokesperson said initial reports suggest that the teenager was in a trench being dug for pipes when a large slab of concrete from a driveway fell down, pinning him in an almost standing position.

An excavator was used to lift the concrete and relieve the weight then a crane lifted the concrete off him.

He has now been freed and has been taken to St George Hospital in a serious condition with suspected leg fractures.