The center of the Scottish capital Edinburgh was brought to a standstill on Monday as hundreds of anti-capitalist demonstrators and anarchist groups clashed with police in a series of protests against the G8 summit.
A stand-off between police and a core of 300 protesters continued in the evening as mounted police and officers dressed in riot gear corralled them in Princes Street, Edinburgh's main shopping thoroughfare.
Police blamed "a hard core of determined activists" intent on causing trouble for the scuffles. They said the protesters were highly organized, using maps, radios and mobile phones to plan their actions to cause maximum disruption.
Weapons were seized from some protesters and some police officers were attacked with sticks and staves. Police said "key ringleaders" were arrested.
Police and protesters clashed at several locations across Edinburgh. In one confrontation in Princes Street Gardens, police drew their batons and forced back people who were throwing missiles.
A small group of demonstrators, who had their faces hidden behind black scarves and are believed to be part of the Black Bloc hardline European anarchist group, then began attacking police officers.
Injured officers retreated to the other side of the street, some bleeding from cuts, others caked in mud after protesters tore up plants and flowers to use as missiles.
Last night the violence intensified across Edinburgh.
Violent clashes broke out between police and a group of about 1,000 protesters in Rose street as a faction began throwing missiles.
Police drew their batons and charged, dispersing protesters into side streets.
At other locations protesters were seen digging up cobbles to throw at police and several tried to pull down fences to make improvised weapons.
Locals on their way home from work were caught up in the clashes.
Monday's protest was organized by the umbrella group Dissent. It is believed to be the first of a series of protests designed to cause disruption in Edinburgh and around Gleneagles, the summit venue, north of the city.
One anarchist, who declined to be named, said: "This is just the start of it. Nobody wants to get arrested ahead of Wednesday. We've got a lot more planned."
Police said they had intelligence that trouble will intensify in Edinburgh as the week progresses.
Another protester, Jay, 20, from Edinburgh, speaking from the top of a bus shelter, said the authorities had made the situation worse by warning that protests could get ugly.
"They've spent all week making it out like everyone coming up is terrorists and anarchists. And now the families have left after the weekend protests, the police are a lot more hardline. If there's fighting like this, people are going to get hurt," he said.
Anarchists from Italy, Spain and France are all believed to have taken part in yesterday's protests.