The boy, believed to be 11, was stabbed in the neck during a skills camp involving dozens of young players at a pitch in the Topsail neighbourhood of Conception Bay South, a suburban town just west of St. John's.
Witnesses said the teenager, believed to be 18, ran from the bleachers and stabbed the boy.
"He fell into my arms, and I just grabbed and held his wound, and put my arm around the back of his head and lay him down on the field and lay with him, and just applied pressure to his wound," said a soccer father who was on the scene, whom CBC News is not identifying for privacy reasons.
"I just screamed to some other parents to call 911."
The boy was rushed to hospital and had been listed in serious condition. His status was not known Friday morning.
The Royal Newfoundland Constabulary arrested a suspect a couple of hours later in another neighbourhood.
Police Chief Bill Janes said he could not divulge details of how the suspect — whom he described as "a male youth" — was taken into custody.
'Disbelief that someone could do that'
Conception Bay South Mayor Ken McDonald said the stabbing has shocked people in his community.
"Fear and shock are a couple of words to describe it. To me it, it was a bit of disbelief that someone could do that," said McDonald.
McDonald said counsellors are available to help children, parents and coaches who saw the gruesome scene play out before their eyes.
"I'm sure it's very traumatic. You watch an incident like this happening on a TV show and it kind of gives you the shivers — to be there first hand and see it happen? It's not going to be easy to put that out of their minds in the very near future," said McDonald.
Janes was visibly shaken as he spoke late Thursday night with reporters at police headquarters.
"This incident can only be described as a terrible tragedy," he said.