Mountain bikers were directed into traps laid by saboteurs that included wire strung at head height across the course - which could have decapitated a racer.
The organiser of a time trial in the Mournes believes disgruntled hill walkers - unhappy at forest paths being used for such events - may have been behind the attack.
Glyn O'Brien from Newry, who ran the Vitus First Tracks Enduro Cup at Kilbroney Park in Rostrevor, slammed those responsible for putting wire and string at head height across downhill sections where riders can reach speeds of up to 40mph.
Ahead of the time trial, rocks and logs were also dislodged and placed on the track.
Mr O'Brien, who took part in yesterday's event, said fortunately the traps were spotted.
Police are investigating. A PSNI spokesman said: "This was an unbelievably dangerous thing to do. Whoever is responsible could have caused great injury to those participating had the route not been checked".
In yesterday's event, 200 bike riders hurtled down the Mourne Mountains.
Mr O'Brien said it went well and that all competitors were all still happy to take part.
He said the disruption took place overnight on Friday into Saturday ahead of practice sessions. There were no further incidents before the racing.
Mr O'Brien said it was disturbing to hear from other downhill bikers that similar incidents happen regularly in places like Cavehill in Belfast and Tollymore Forest Park in Newcastle.
He said: "There were five or six places where they had put the wire/string at head height and they had used our tape to re-direct riders into areas where the wire was. It is very thin and you wouldn't see it until the last minute.
"By the look of it I don't think it was kids who did this but this time I have a feeling it was maybe walkers who don't like bikers, who maybe feel mountain biking shouldn't be on those trails.
"I have heard of things like this happening but it is the first time it has happened at an official event, but it won't put us off next year. It is an annual event and it is a brilliant venue."
He said the Rostrevor race was officially organised with permission from the authorities and competitors came from across Ireland, Great Britain and even Australia.
He said a marshal noticed and was able to ensure the first riders were stopped.
"For the event the riders were happy to take part after everything was checked by the marshals, but the scary thing is what is to stop that person coming out again and doing the same thing when there are no marshals?"
He said marshals will now be briefed to check courses across Northern Ireland and searches of the tracks will take place before an event at Ballycastle Forest in August.
Mr O'Brien said the Rostrevor event contributes a significant amount of money to the area and he hoped it would "not create a negative vibe" and keep people away.