Seventeen people have been injured, some seriously, after a crash involving two cars in Stevenage.
Hertfordshire Constabulary said the cars were involved in a collision on Monkswood Way at about 9.45pm on Thursday.
A spokeswoman said: "There were a number of people in the area at the time and 17 people are presently being treated for injuries, some of which are serious."
Emergency services are expected to remain at the scene for some time.
Police have urged any witnesses to get in touch and have asked anyone with footage of the incident to retain it.
MailOnline reports two 'boy racers' ploughed into crowds of car enthusiasts watching them race.
Ciaran O'Connor, 33, who witnessed the crash told Mirror Online: "You could just see kids on the floor not moving. I saw at least 10 people injured. One boy had quite a bad arm injury.
"Another boy had his arm cut open and his mum was really upset. She came 15 minutes after the incident and grabbed him and hugged him.
"The first car hit three or four people. I think the slow car went into eight to nine people but it was going slow.
"There were about 200 people there. The majority of them were 15, 16 or 17."