Seventeen people have been injured, some seriously, following a "horrendous" crash involving two cars in Stevenage, police confirmed.
The cars were involved in a collision on Monkswood Way at around 9.45pm on Thursday, during what has been reported as car meet event.
Witnesses described on social media how a car smashed into a crowd of at least 10 people who were watching “boy racers” on Thursday night.
Footage showed a number of cars speeding up and down the street as dozens of people watched on.
A witness said a crowd of young people "about 18 years old-plus" were involved in the crash.
Ambulances were seen lining the road following the collision.
Hertfordshire Constabulary said 17 people were being treated for injuries, some of which are serious.
It said two cars were involved in the smash.
A spokeswoman for the force said: "There were a number of people in the area at the time and 17 people are being treated for injuries, some of which are serious."
The spokesman added: "I am not aware of any fatalities or arrests at this stage, however investigations are ongoing.
"We are working with the road policing unit and local policing teams for further updates and will be in touch in due course."
One witness wrote on Twitter: "Horrific incident in Stevenage. A car (boy racers) has plowed [sic] into at least 10/15 people on the side on a road who were watching on.
“Must be 30+ emergency services on the scene. Thoughts are with all involved, looks so bad.”
Witness Ciaran O'Connor told Good Morning Britain: "We turned round the roundabout and as we turned around what faced myself was a car speeding and mowing into some kids in the middle of the road.
"We looked around and there as kids obviously injured and not in a good way. It was just a scene of absolute chaos."
— Richard Gaisford (@richardgaisford)July 19, 2019
Another tweeted: "I've just witnessed that horrendous crash in #Stevenage, no more than 50ft away from me. I'm still trying to process it all."
A group called Cruise-Herts had said on Facebook that it was holding a car meet event on Thursday to raise money for a charity supporting bereaved parents.
Rix Sidhu, of the club, said the event had been running regularly for 17 years, providing a "safe space" for car enthusiasts to meet within a controlled, closed environment.
He said: "We are devastated. I've been running this for 10 years and we have never had one incident.
"We were raising money for charity, as we often do, and there was no speed element to it.
Seems clear that those attending the Hertfordshire car cruise event didn’t stay static as organisers asked. Car parks show evidence of drivers doing “donuts” and stunts before the horrendous accident. pic.twitter.com/X1pJSTHBIO— Richard Gaisford (@richardgaisford)July 19, 2019
"We held the meet in a car park with a speed bump at the entrance. But unfortunately some people went a bit rogue.
"We try and stop that, we urge people - urge them on social media beforehand - not to go out on the roads, not to risk injury or anything.
"But unfortunately, in this age of social media and Snapchat, people want to get footage and post things to their friends, which seems to drive some people to the main road."
Mr Sidhu added that he would "never hold the event again".
"Our thoughts go out to the victims involved and thank you to those who helped," he said.
Mr Sidhu added: "We have a liaison officer to pass number plates to police if there is ever any trouble; we try to make it as safe as possible.
"Some people like drinking and that sort of thing, we just like cars. They are our pride and joy, what we are interested in.
"Unfortunately this incident means we are all going to be tarred with the same brush - people will say we are boy racers."
He added: "There were several younger people in the crowd, but they weren't kids. They were about 18 years old-plus."
The East of England Ambulance Service said it had taken 12 people to three hospitals - Lister Hospital in Stevenage, Watford General Hospital and The Princess Alexandra Hospital in Harlow.
An air ambulance also attended the incident.
Police have urged any witnesses to get in touch and have asked anyone with footage of the incident to email it to OPtephra@herts.pnn.police.uk.