Police have launched an investigation after hundreds of cyclists competing in a Perthshire charity race were forced off the road to mend punctures after several miles of the route were strewn with carpet tacks.

It is believe that local protesters, upset by the disruption caused by the Etape Caledonia event, were responsible for the sabotage.

The 81-mile cycle race, now in its third year, had attracted around 3500 competitors from all over the world yesterday, but police stepped in to halt the contest on the Schiehallion mountain road for an hour after a number of competitors suffered flat tyres after cycling over the tiny nails en route to the summit.

The downward slope also had to be cleared of the tacks before the race could continue amid fears that a "treacherous" combination of sudden punctures during high-speed descents could result in a "massive pile-up". In total, it is estimated some 10 miles of road were targeted.

A major attraction of the Etape Caledonia is that it takes place entirely on closed roads - the only cycle event in Britain to do so. However, this has prompted protests by some locals angry at the disruption the one-day blockades cause to motorists in the local community, particularly those trying to make their way to church.

The race starts and finishes in Pitlochry and takes in Loch Rannoch, Schiehallion and Comrie Bridge on the way.

Organisers IMG were alerted to the problem around two hours into the race yesterday, at 9.30am. They said they believed "local protesters" were responsible for the action, which Tayside Police described as "reckless and irresponsible". The force is appealing to anyone with information to come forward.

The annual race was expected to raise around £225,000 for Macmillan Cancer Support, and also brings millions of pounds in tourism to the Perth and Kinross area.

Kathy James, who runs a bed and breakfast in Aberfeldy, said she felt disgusted that anyone would spread the road with tacks.

She said of the incident: "It disgusts me as a local; there's no logic behind it. There's been a lot of people coming off their bikes. "

Kyle MacRae, who travelled from Glasgow to participate, had a puncture after police had already tried to brush the tacks off the mountain road.

However, he described the support from locals lining the route as "fantastic", adding: "People were clapping and cheering all along the route and we were made to feel very welcome. An event like this has to be good for the area. To have it sabotaged like this is terrible...putting tacks on the fast descent is insane, no matter how strongly you feel about our closed-road event, there's no excuse for that."

Clive Bowman, 40, participating in the race for the second time, said he had "never seen so many punctures". Speaking to The Herald about 200 metres from the finish-line, he said: "I've seen about 300 to 400 people at least stopping to mend punctures. It's been very disappointing for people who wanted to compete, but it's been a nice day and very well organised.

"This has just put a bit of a downer on things for us."

Describing the vandalism as "small-minded", he added: "I've taken part in events all over the place, but the fact that the road is closed is tremendous; it makes the cyclists' experience so much better.

"It's just four hours on a Sunday morning once a year. There are bigger things in life to worry about."

Richard Coombs, 43, had travelled from Bristol to take part with friends, one of whom suffered five punctures. He said: "We came here to do an 81-mile race and we didn't do it; it was cut to 72. But it was brilliantly organised, it's just a shame somebody had to spoil it."