More than 100 activists, many covering their faces with bandanas and wearing hoods, streamed from a makeshift campsite in Stirling, central Scotland, early on Wednesday where about 5000 anarchists and anti-globalisation protesters are staying.
An Associated Press Television News cameraman said he saw a group of about 100 people smashing the windows of parked cars and throwing stones at police.
A spokesman for the Central Scotland Police confirmed that officers had come under attack.
In body armour, helmets and carrying shields, police officers formed a chain across the M9 highway - the main approach route to Gleneagles from the Scottish capital, Edinburgh - as dozens of protesters ran along the closed highway.
Many ran up an embankment and escaped across fields when they got to the police line.
In nearby Bannockburn, protesters - some wearing black crash helmets and carrying iron bars - smashed the windshields of parked cars and threw rocks at police vans.
Police and protesters clash in
Scotland ahead of G8 talks
A group linked their arms through inflated tire inner tubes and charged a line of riot police blocking the road.
Several attacked a police van, hitting it with iron rods and kicking the headlights as the vehicle reversed down a street.
Demonstrators pulled a protective iron grille from the windows of a local Burger King restaurant and smashed the glass.
Police cancel protest
Police cancelled a planned protest on Wednesday near the site of the G8 summit, saying it was too dangerous after earlier violent clashes between activists and police.
"There has been a lot of disruption, a lot of criminal activity. It is too dangerous to go ahead," said a spokeswoman for Tayside police, tasked with protecting leaders at the Gleneagles golf resort.
Vow to disrupt
Anti-globalisation campaigners have vowed to disrupt the summit of the leaders of the Group of Eight industrialised nations, who meet later on Wednesday at the tightly secured Gleneagles hotel near Stirling.
British Prime Minister Tony Blair, who this year leads the G8, arrived in a helicopter at the hotel, after flying back to Britain from Singapore where he lobbied for Britain's 2012 Olympic bid.
In Edinburgh, several small groups of demonstrators, also wearing black, roamed the streets. Police carrying shields formed a protective line around the Sheraton hotel.
Police have cancelled a planned
protest near the summit site
Later on Wednesday, activists planned a demonstration in Auchterarder, just north of Gleneagles. About 40 buses carrying protesters were due to leave Edinburgh for the city.
Protests were also held in Indonesia and Hong Kong. Indonesian students held hanging baby dolls to symbolise even babies have a responsibility to international debt.