An Audi A4 hit the area outside Streets nightclub, in John Street, Porthcawl, Bridgend county, at about 01:00.
The injured were aged between 17 and 43, with nine taken to hospital in Bridgend and four to Swansea - most with leg injuries.
A 24-year-old man has been arrested on suspicion of dangerous driving.
An 18-year-old woman is helping South Wales Police with their inquiries. Both have been taken to Bridgend police station.
It is understood that seven men and six women in total were injured.
The Abertawe Bro Morgannwg University Health Board said three of those injured and taken to the Princess of Wales Hospital in Bridgend have been treated and discharged.
The remaining six people have sustained serious injuries but are in a stable condition.
Those taken to Morriston Hospital in Swansea include two 18-year-old men, a 21-year-old woman, and another woman who is 38.
Two are in a serious but stable condition, while the other two were admitted as a precaution.
Supt Andy Valentine said: "This was declared a major incident by the police and ambulance services. All three emergency services were deployed to the incident and worked seamlessly to provide a fast and efficient response."
Nick Smith, of the Welsh Ambulance Service, confirmed: "The injuries were all to lower limbs with no-one reported in a life-threatening condition."
A spokesman for the health board said additional staff were called in to hospitals to deal with the incident as it unfolded.
"Staff have been excellent, and didn't hesitate to come in. They worked efficiently and in an organised way, and they helped considerably during a difficult situation."
Thirteen people have been taken to hospital after a car crashed into a smoking area outside a nightclub.