Two have fallen from a window cleaning platform in central Sydney, emergency services say.

The men were cleaning windows at a building on Pitt Street near the Bridge Street intersection when a cable snapped on their platform just after 11am on Tuesday, witnesses said.

Both men were taken to St Vincents Hospital in a serious condition, a NSW Ambulance spokesman said. One man, in his 20s, suffered back and internal injuries; the other man, in his 50s, serious leg injuries.

The platform was believed to have been about 12 storeys up when they fell.