A group of vigilantes, in off-road vehicles, have blocked school buses and cars from taking runners from Marrawah Hall to the start of the takayna Ultra.
Police are currently on the way to Marrawah.
About 100 runners from across Australia and overseas are never the less being ferried to the start point from outside the blockade.
"While this illegal behaviour will bring an undeserved negative reputation to North West Tasmania, it has failed to stop our foundation's run for the takayna/Tarkine," Bob Brown said.
"Yesterday Premier Hodgman was, like us all, talking tourism in Circular Head. Today an organised group of malcontents is wrecking that reputation," Brown concluded.