Police are searching for a violent gang that viciously bashed a 26-year-old woman until she lost consciousness on a crowded NightRider bus in Melbourne’s city centre.

Victoria Police spokeswoman Anita Brens said the brutal attack happened on the late-night bus service, bound for Frankston, during the early hours of Sunday morning.

The bus was parked on Swanston Street about 2.30am when a group of six or seven men and two women boarded the packed bus and began acting aggressively.

A number of concerned passengers, including a 26-year-old St Kilda woman, appealed for the bus driver to remove the offending group from the bus.