Just hours after Scottish police, fearing violence, cancelled an official protest march on the outskirts of the G-8 summit venue, they announced it would proceed regardless.

A short while later, thousands of protesters began their noisy march towards the fenced-off perimeter around the site of the three-day meeting.

Earlier Wednesday, after protesters destroyed property and cut off traffic on one of the main roads leading to the summit, Tayside Police said the march planned to start in the village of Auchterarder would have to be called off.

March organizer G8 Alternatives was quick to respond with outrage.

Accusing the police of "disgraceful behaviour," spokesperson Gill Hubbard said the decision was misguided, and made without consulting her group.

"This is a serious indictment of British democracy,'' she said.

Within hours, Tayside Police relented. The march planned for the village of 4,000, just four kilometres from the Gleneagles resort, could go ahead.

Auchterarder had been sealed off temporarily, after protesters attempting to make good on their promise to disrupt the G-8 summit clashed with police guarding the summit venue.

The main highway leading to the Gleneagles Hotel was closed, as protesters took to the road on foot. A few dozen protesters reportedly formed a blockade on the M9 artery, forcing police to divert traffic.

The Associated Press reports that a group of as many as 200 people threw stones at police and smashed car and shop windows, as they tried to approach Gleneagles from the campsite in Stirling that's become the temporary headquarters of thousands of protesters.

There are reports nine police officers were taken to hospital, and as many as 60 people arrested in the clashes.

Protesters in the nearby town of Bannockburn, some brandishing iron bars, also wreaked havoc. A police van was attacked and a Burger King restaurant window smashed during a demonstration.

And in the town of Crieff, where American G-8 delegates are believed to be staying, dozens of protesters used tree trunks and branches to barricade a bridge.

Anticipating the worst in the Scottish capital of Edinburgh, at least 200 police cordoned off the main financial district. Area workers were the only ones being allowed into the district that's home to several major bank and insurance company headquarters.

Several hundred protesters turned back from Auchterarder earlier in the day, massed in downtown Edinburgh. Some blocked traffic on the city's main commercial thoroughfare, while others staged a sit-in before a fleet of buses.

According to Guy Taylor of the group Globalize Resistance, however, the tight security throughout the area was unnecessary.

En route to the march in Auchterarder, Taylor condemned the morning's violence in Stirling as "stupid."

"The message of the march is that the G-8 should not have the power to decide what goes on in the rest of the world," Taylor told AP before he set off early Wednesday.

"The only thing that unites the G-8 is wealth, and they are there to protect that wealth."

Live 8 organizer Bob Geldof, who has called on a million people to take part in a "Long Walk to Justice" in Edinburgh later Wednesday, was also quick to point out the difference between the violent protesters and supporters of his Make Poverty History campaign.

"These are our people,'' Geldof told reporters in the Scottish capital. "You must not conflate the two. Some come in peace and dignity and respect, some just come to make trouble. There is no similarity between them.''

Talking to reporters at a press conference in Gleneagles, U2 frontman Bono delivered a similar message.

"There's two alternatives," Bono said. "Out on the streets, very angry. Or, making your case and organizing in a boring old-fashioned way.

"We have done it and we must not let our people down."

Watching developments from the site of the G-8 conference in Gleneagles, Gerry Barr of Make Poverty History Canada says violence is just a small part of the "mixed bag" of demonstrations now underway.

"There are protests which are partly confrontation and partly carnival," Barr told Canada AM. "And there are those who are lobbying hard to try to ensure that the national teams get to the mark."

Barr is part of the latter group, pushing for leaders to commit to the 0.7 per cent of GDP target for foreign aid first proposed by former Canadian prime minister Lester B. Pearson.

"World leaders have been to the brink on this question several times, but they have rarely done anything of real substance," he said, suggesting Prime Minister Paul Martin's reluctance to make the pledge is threatening to rub off on other G-8 leaders.

"Germany appears to be hiding behind Canada's reticence with its commitment melting away," Barr warned.