A rescue operation is under way in the City Loop after a woman on a mobility scooter fell onto the tracks at Parliament station.

Police, paramedics and firefighters have been called to rescue the woman, aged in her 60s, who fell from platform 3 just before 1.30pm.

The Sunbury, Upfield and Craigieburn lines are running direct to Flinders Street instead of through the City Loop.

Ambulance Victoria spokesman John Mullen said it appeared as though she has sustained a broken leg.

"She's being treated at the moment," he said. "It's going to be a long process to get her off the tracks."

A Public Transport Victoria spokesman said emergency services were controlling the site.

"Please listen for announcements, as some services may be altered at short notice," he said.

Go to the PTV website for the latest updates.