Several people were detained in Edinburgh, and at least two police officers were injured in clashes, while several people fainted in the crush.
The demonstrations began peacefully as protesters, banging drums and shaking bells, marched and danced into waiting police containment cordons in Edinburgh's financial centre.
The demonstrations were part of an array of protests that began on Saturday with a 200,000-strong march through the city calling for an end to poverty in the developing world, especially Africa.
Leaders of the Group of Eight industrialised nations meet amid tight security from Wednesday to Friday at the nearby Gleneagles Hotel, tasked with tackling African poverty and devising a strategy against global warming.
On Monday, black-clad and masked members of the Black Bloc, an anarchist group based in Germany and Scandinavia that has been prominent in protests at past G8 summits, mingled with other demonstrators dressed as fairies and clowns.
Trouble began when about 30 members of the Black Bloc, among about 200 protesters trapped between lines of police, pushed and scuffled with police officers.
"It is vitally important that people make the link between the industrial war machine and the poverty that so many people are suffering from around the world," said protester Jenny Gaiawyn, 26.
Among the activists were people wearing military fatigues, many with painted faces and red noses, who described themselves as the "clandestine insurgent clown rebel army".
"We're anticapitalists, we're for trade justice," said a woman with a painted face, who called herself General Lovely.
Protesters dressed as clowns
said they were anticapitalists
One protester held up a banner outside Starbucks saying, "abolish capitalism before it kills the planet".
About 10,000 officers from across the country are policing the event.
Mindful of the violence that erupted at previous G8 summits in Genoa, Italy, in 2001 and Evian, France, in 2003, Scottish police have been taking no chances.
Around the city, mounted riot police and convoys of police vans with sirens wailing and lights flashing sought to damp down anarchist street carnival demonstrations.
At the Clyde Naval Base in western central Scotland, about 450 demonstrators blocked the road, vowing to shut down the Trident submarine base for the day.
The Trident protesters, who stage the event every year, enjoyed increased news coverage this year because of the big media presence in Scotland for the G8.
Africa and poverty
Earlier on Monday, more than 50 African activists filed through Edinburgh's airport snapping their fingers every three seconds to symbolise the death of a child from extreme poverty.
The Africans were among thousands of people converging to demand action for Africa from the world leaders.
"It is loud and clear," said Wahu Kaara, of Kenya.
Nobel peace prize nominee Wahu
Kaara: Africa has refused to die
"Enough is enough. This is the moment for the world leaders to think seriously about life and not always to think about profit.
"Africa has refused to die," she said. "Africa wants to live."
The Africans were among a group of 150 campaigners from countries including Cambodia, Afghanistan and the United States who met in London and flew together to Edinburgh as part of an effort called the Long March to Justice.
Bob Geldof, organiser of the Live 8 shows held around the world on Saturday, saw the flight off from London's
With officials from the G8 group of wealthy nations negotiating furiously ahead of the summit that starts on Wednesday, Geldof said Britain was pushing hard for a deal that would help put Africa's economies back on their feet.
"I am not sure the others want to do it, which will be a grotesque failure," he said. "I don't think they are anywhere near there."
South African Kumi Naidoo, head of the Global Call to Action Against Poverty, said it was crucial that the G8 leaders heed Prime Minister Tony Blair's call for a major boost in aid to Africa, forgiveness of impoverished nations' debts and reform of the global trade rules which advocates say stack the deck against poor countries selling their goods overseas.
Naidoo said a deal agreed last month by G8 finance ministers to cancel $40 billion of debt owed by 16 of the poorest nations was "a small, belated step in the right direction".
Activists say poverty and hunger
are weapons of mass destruction
Naidoo said more than 60 countries should have their debts written off. He also worried that the write-off would come with possibly harmful economic conditions.
"Debt needs to be written off not as an act of charity but as an act of justice," he said.
Many of the outstanding loans were made by the West to corrupt, dictatorial governments that lenders hoped to woo for strategic reasons during the Cold War, Naidoo said.
He also argued that because rich countries benefited so much from colonial exploitation of Africa - including slavery - they owe the continent a great deal in return.
He said he feared negotiators were making little progress ahead of the summit on what he views as one of its most important priorities: overhauling rich countries' agricultural subsidies and trade barriers which make it hard for African goods to reach Western markets.
"We're not going to have a peaceful and sustainable world if we do not have a breakthrough in poverty"
head, Global Call to Action Against Poverty
"We are deeply concerned that there's been absolutely no movement" on trade, he said.
He said he was sure the G8 could make a major difference for Africa if it summons the political will to act. America and its allies spent hundreds of billions of dollars on the war in Iraq, he said. They could better lives in Africa with far less and must do so soon, he said.
"Poverty is a weapon of mass destruction, hunger is a weapon of mass destruction," Naidoo said. "We're not going to have a peaceful and sustainable world if we do not have a breakthrough in poverty."