The accident happened in Leeds, a city in northern Yorkshire county, on Saturday night.
A 12-year-old, two 15-year-olds, a 24-year-old and a 28-year-old died in the smash, West Yorkshire Police said, while two 15-year-olds are in police custody.
Two 15-year-old boys are in police custody but police have not released any details as to why they are being held.
The families of the dead have been informed.
Police were called to reports of a crash on Stonegate Road in Meanwood just before 10pm last night.
It tweeted: "Police dealing with a serious road traffic collision, Stonegate Road, Meanwood, Leeds".
Anyone with any information about the crash is asked to contact West Yorkshire Police.
Two 15-year-old boys are being held in custody on suspicion of unsafe driving.
Earlier police said they were working on the assumption all seven were in the same vehicle - a Renault Clio.
Detective Chief Inspector Jim Griffiths said: "This is clearly a tragic incident in which five young people have lost their lives".
Stonegate Road is expected to remain closed for much of Sunday.
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