Three people were seriously injured following two incidents where motorbikes ploughed into the crowd.
North Yorkshire Police said a dozen people were injured in both crashes.
Six people were injured in the first crash, which occurred at 10.45am yesterday (Sunday September 24), and one person sustained serious injuries.
Air ambulances were called to the Scarborough race track and two people were taken to James Cook Hospital in Middlesbrough and one to Scarborough District Hospital.
The other three injured were treated at the scene. The rider of the motorbike was also taken to Scarborough Hospital but was later discharged.
A second crash, which happened at the same point on the circuit, occurred around three hours later.
Another six people were injured including two seriously with one being taken to hospital by air ambulance.
One was taken to James Cook Hospital and three were taken to Scarborough District Hospital. Two were treated at the scene. The rider of the bike was checked at the scene.
Four air ambulances were called to the racetrack following both incidents.
Racing was cancelled following the second incident.
In a statement, the circuit said: "Today we have had two serious incidents involving a number of spectators.
"Our medical personnel, together with the marshals and officials of the club, local police, NHS, ambulance staff and helimeds have treated all the injured.
"In view of the seriousness of these incidents and our requirements to call on outside assistance to help with this treatment, we have reluctantly taken the advice of everyone concerned and have decided to cancel the rest of the meeting.
"We sincerely apologise to you all for taking this decision and hope you will understand our difficulties in this unprecedented situation.
"We wish you all a safe journey home."
One of the riders, Jamie Coward, from Hebden Bridge, tweeted his apologies after crashing through the fence at Drury’s Hairpin during morning qualifying.
He said he was "very upset and shaken by what had happened", adding: "Really sorry to spectators involved in my accident today there was nothing I could have done."