An eyewitness told CBC News that roughly 100 kids were on the soccer field in the Conception Bay South-area field at the time of the incident as part of a soccer camp, when an older boy, about 18, ran from the bleachers and stabbed the young boy in the neck.
The Royal Newfoundland Constabulary said the boy is in serious condition.
An eyewitness said people, including many parents who were watching from the stands, were in shock and started screaming and yelling. Although some people tried to stop the attacker, he got away.
One witness said the young boy was holding his injured neck. The witness said he applied pressure to the boy's wound while waiting for emergency responders to arrive.
The coaches got the children off the field and into the dressing rooms in lockdown mode while they waited for police to arrive.
Witnesses reported seeing the suspect flee the scene in a vehicle.
RNC Chief Bill Janes confirmed later Thursday night that police have a suspect in custody, and officers had seized a vehicle from the area of the incident.
Police said they will not be releasing any information about the suspect or how he was apprehended, and cannot say if the suspect and victim were known to each other.
Janes said a stress management co-ordinator has been called in to help victims and witnesses deal with the situation.