A major crash involving more than 20 vehicles has caused traffic chaos north-west of Melbourne, with paramedics treating multiple people at the scene.

The multi-vehicle collision on the Western Freeway at Myrniong near Bacchus Marsh happened just before 4pm on Monday.

Victorian Police said more than 20 vehicles were involved in the crash that happened with heavy fog in the area.

Tweet from @AmbulanceVic

Ambulance Victoria confirmed paramedics were responding to “multiple multi-vehicle incidents” at Myrniong with crews treating at least 18 patients as of about 6pm.

Police said they had no reports of serious injuries.

Two people were taken to Ballarat Base Hospital and the incident was ongoing, an Ambulance Victoria spokeswoman said.

VicTraffic said the Western Freeway was closed in both directions between Bacchus Marsh and Ballan.

-AAP