A witness wrote on Facebook: "Horrific incident in Stevenage. A car (boy racers) has ploughed into at least 10/15 people on the side of a road who were watching on.
"Must be 30+ emergency services on the scene. thoughts are with all involved, looks so bad."
Ciaran O'Connor, 33, who witnessed the crash, reportedly said he saw three or four young people lying in the road.
"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," he told Mirror Online.
He said one of the cars was travelling around 50mph as it hit the bystanders on the central reservation.
"They went flying. A car pulled out of a retail park and this other car, doing about 50 miles-per-hour, basically dodged it and took these kids out," he was quoted as saying.
One witness said on Twitter: "I've just witnessed that horrendous crash in #Stevenage, no more than 50ft away from me. I'm still trying to process it all."
Police have urged any witnesses to get in touch and have asked anyone with footage of the incident to retain it.
The accident happened during a weekly meet of the modified car club Cruise Herts.
A post on the group's Facebook page read: "This Thursday night meet is an extra special meet we as a group are raising money for the excellent charity of 4Louis who help supporting families through the heartache of miscarriage stillbirth and child loss.
"This is a Strictly Static Meet & Any Antisocial Behaviour Will Not be Tolerated! Any Such Behaviour will be Dealt with by the Local Authorities!! So strictly NO Wheel Spins NO Burnouts and NO Drifting."
Rix Sidhu, 29, of the Cruise Herts club, said the group were not boy racers but "car enthusiasts".
"We don't condone street racing, we organise static meet-ups," he told MailOnline. "We park up and walk about and admire the cars, we've even had families come in the past. We've been doing this for 10 years.
"But we can't control everyone and we don't promote this. We are not boy racers, we are car enthusiasts."
At least 14 people have been injured, some seriously, after two cars crashed and ploughed into bystanders at a car meet in Stevenage.
The accident took place on the A602 in Stevenage, Hertfordshire, on Thursday evening as hundreds of people watched the event.
Footage posted on social media showed a car emerging onto the main road while another vehicle was passing at speed. They collided, causing both cars to hit bystanders on the side of the road and on the central reservation.
Hertfordshire Constabulary said two cars were involved in a collision on Monkswood Way at about 9.45pm.
A spokeswoman said: "There were a number of people in the area at the time and 14 people are being treated for injuries, some of which are serious."
Emergency services are expected to remain at the scene for some time.