Several ambulances were sent to the scene along with police, the fire and rescue service and the coastguard, after the vehicles shifted on board the ferry, causing damage.
No one is reported to have been injured in the incident, which happened at around 7.30am on Tuesday.
P&O Ferries said in a statement: “We can confirm that an incident took place on the European Causeway this morning.
“In extreme weather conditions, a number of vehicles have moved position on the decks causing damage onboard.
“All passengers and crew are safely accounted for, there are no reports of injuries and the emergency services are in attendance.”
A Scottish Fire and Rescue Service spokesman said: “The Scottish Fire and Rescue Service was requested at 7.32am on Tuesday December 18 to assist our emergency service partners with an ongoing incident involving vehicles upon a ferry berthed at Cairnryan Ferry Terminal, Wigtownshire.
“Operations control mobilised four fire engines to the scene and firefighters presently remain in attendance.”
A Police Scotland spokesperson said: “Shortly after 7:40am, we were notified of a number of vehicles having overturned on the P&O European Causeway Ferry while sailing from Larne to Cairnryan.
“A multi-agency response was implemented and emergency services attended Cairnryan port, awaiting the ferry.
“The ferry is now docked safely at Stranraer and Scottish Fire and Rescue Service, ambulance crews and Police Scotland are in attendance.
“During a choppy sailing, there has been a movement of vehicles on the car deck and this has caused around six vehicles to tip over onto their side.
“Due to the movement on the deck, there are a number of passengers confined to vehicles.
“We are able to confirm all 52 passengers and 56 crew on board are safe and well and no-one has any injuries.
“The incident remains ongoing and our emergency services will remain at the ferry terminal to provide any necessary support for the time being.”