Thumb tacks were deliberately thrown across the road during yesterday's Around the Bay, puncturing the tyres of up to 12 cyclists who have blasted the stunt as dangerous and "revolting".
Cyclists in the 210-kilometre event around Port Phillip, Australia's largest bike ride, were nearing Rosebud when their tyres began popping about 3.30pm.
Simon Kennedy said one of his cycling group received a puncture, while up to 12 others in nearby riding groups also had to stop and fix their pierced tyres.
They discovered another 20 thumb tacks scattered across the road and cleared them away.
"Everybody was having to fix their wheels and we found probably 20 odd drawing pins on the road," he said.
"It was clearly deliberate, which is outrageous.
"If you get a very fast puncture like that people can come off. It's just a revolting thing to do.
"Overall it was a great day, but this was just ugly."
Around the Bay is Australia's largest bike ride, with more than 16,500 people participating last year.
Bicycle Victoria spokesman Garry Brennan said similar incidents were reported each year.
"This happens every year in the same location, between Rosebud and Frankston," he said, adding that it was not known who the culprits were.
"It's disappointing that someone tries to deliberately spoil a great day out for so many people."
Mr Kennedy said it was another example of aggression towards cyclists.
"There is a level of aggression towards bike riders that sometimes comes out in people," he said.
"Most people are fine of course but there are a percentage of people who consider it their right to try to knock people off or scare them or shout abuse. This is just the really thin edge of the wedge."