The tacks burst hundreds of tyres along sections of the 81-mile course
Police are investigating after carpet tacks were spread across roads bringing a major cycle race to a halt.
More than 3,500 people were taking part in the Etape Caledonia, over 81 miles around Pitlochry in Perthshire.
It is the only cycle event in Britain where all the roads travelled on are closed - which has angered some locals.
Tacks were strewn on a section of the race, bursting hundreds of tyres. Police said it was a reckless act and it was fortunate no-one was hurt.
Cyclists ended up with damaged tyres and the event had to be stopped but organisers cleared the road and the race was completed.
The winner Veli-Matti Raikkonen - who is originally from Finland and is a member of Aberdeen's Granite City racing team - was one of those who suffered a puncture after riding over the tacks.
The attack on the race brought cyclists to a standstill/Pic: Richard Thomas
A spokeswoman for the race said organisers were now working with Tayside Police to discover what had happened.
She said: "The obstruction is a safety matter and it is one the organisers take very seriously."
The lead group of riders suffered punctures on a section of road at Innerhadden and Schiehallion - about 43 miles into the course - and hundreds of other cyclists suffered punctures as they approached the area.
The spokeswoman added: "It is really not acceptable that this kind of behaviour is allowed to happen.
"The organisers identified the area, stopped the cyclists who were coming up to it to minimise the number who were affected, and they also dealt with the punctures very, very quickly."
A spokesman for Tayside Police said: "It was fortunate no-one was injured as a result of this irresponsible behaviour. Uniformed and detective officers are carrying out inquiries in the area and are following a positive line of inquiry."
The event has been running for several years and has developed into the Perthshire Cycling Festival, but has brought protests over the disruption caused by the three-hour Etape Caledonia.
Some campaigners are angry because they believe their freedom of movement is being restricted, visitors may be put off coming to the area, affecting tourism and business, and locals could be prevented from getting to church.
Peter Hounam, from the Anti Closed Road Event (Acre) group, said: "Acre is against the closure of roads for cycle events in our area, we do not object to cyclists or people taking part in events.
Cyclists were left angry and frustrated by the events/Pic: Allen Glen
"We totally deplore anyone taking direct action and we want people to have peaceful protests.
"We don't condone what has happened, but it shows there is real frustration from people who feel the authorities are not listening to them."
He said the event discouraged tourists because of the road closures and that it did not bring the kind of sustainable tourism needed in Perthshire, as many of the participants only stayed overnight.
But Kathy James, who runs a bed and breakfast in Aberfeldy, said the race brings millions of pounds to the area.
She told BBC Scotland: "What they've done is they've spread, from Rannoch down to us, tiny, tiny carpet tacks - I've got one - they've just spread the road with them.
"There's a chap outside who came off his bike, there's been a lot of people coming off their bikes and basically they had to stop the race further up.
"This area relies on tourism, it disgusts me as a local, I just think there's no logic behind it."
Finance Secretary and North Tayside MSP John Swinney also expressed disgust over the incident.
He added: "This is a highly dangerous act which could have brought serious injury to people on the event."
Alun Pugh, a former member of the Welsh Assembly, travelled to Scotland for the race.
He said: "It's just really, really sad because this is the biggest cycling event for amateur mass participation cyclists in the whole of the UK.
"It very, very disappointing indeed that a small number of people have chosen to disrupt and wreck it."
The event raises cash for Macmillan Cancer Support and the charity estimated it would raise £225,000 this year to help fund its work.
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