Leaders and protesters from around the world are drawing out their battle lines ahead of the G8 summit.
Police said there had been a "hard core" of protesters
In Scotland, where the G8 leaders will meet from Wednesday, hundreds of demonstrators clashed with police as they called for fairer global trade.
Police said they had arrested about 30 protesters in Edinburgh.
African leaders, meanwhile, met in Libya to discuss how to capitalise on a high-profile poverty campaign that was boosted by pop concerts at the weekend.
Africa and global poverty are set to dominate the three-day meeting in Gleneagles, which will be chaired by British Prime Minister Tony Blair.
Doubling aid to poor nations and cutting debt are among the summit's declared goals.
Another key topic will be climate change, though US President George W Bush seemed to rule out any new climate control treaty.
Leaders also are likely to take in the impact of both high oil prices and exchange rates on economic stability.
In Edinburgh, police in riot gear and on horseback clashed with several hundred black-clad anarchists and anti-capitalist protesters.
Protesters threw stones, sticks, clumps of grass and drink cans.
Previous G8 protests have ended in riots
Small skirmishes were still going on Monday evening, the BBC's home affairs correspondent Andy Tighe said.
"The agenda in Edinburgh today simply seems to be to disrupt the life of the community," said Chief Superintendent Charles Michie of Lothian and Borders police.
"It's certainly not clear that they are genuine protesters, that they have legitimate concerns."
Demonstrators have hit back, saying the police were heavy-handed and pushed and punched people.
Amid the violence there also were peaceful protests with demonstrators dressed as clowns and some even stripping naked.
"We're anti-capitalists, we're for trade justice," said one woman in a clown costume calling herself General Lovely.
On the west coast of Scotland, about 700 protesters gathered outside the Faslane nuclear submarine base to highlight the fact that wealthy nations are still selling weapons to developing countries.
WHAT IS THE G8?
Group of eight major industrialised states, inc Russia
Canada, France, Germany, Italy, Japan, Russia, UK, US
Originally set up to discuss trade and economic issues
Now leaders discuss global issues of the day
2005 Summit agenda
In Libya, leaders of the 53-member African Union, joined by UN Secretary General Kofi Annan, have been calling for real change, not hollow words.
"Africa will lose hope" if rich nations break their promises to the continent, said Rene N'Guettia Kouassi, the AU's economic director.
UK Chancellor Gordon Brown said on Sunday that G8 leaders had already made "a great deal of progress" on "one or two of the major issues" of poverty reduction.
He said that:
G8 nations have already reached agreement on a doubling of aid for poor countries and on 100% debt relief.
European aid for poorer states was also set to double, with 13 European countries accepting a timetable to increase aid to 0.7% of their annual income.
The US also has been taking steps to address the poverty issue.
President Bush has proposed doubling US aid to Africa over the next five years, lifting it to $8.6bn by 2010, as long as African leaders tackled corruption.
The UK has said it will promote ideas put forward by the Commission for Africa, established by Mr Blair.
The commission recommended 100% debt cancellation for the poorest nations; the doubling of aid to Africa; the removal of barriers to trade with African nations, and improved governance.
US resistance to curbs on greenhouse gas emissions means a breakthrough on climate change is unlikely.
President Bush said the fact that the G8 meeting was being chaired by the UK, his close ally in Iraq, would not in itself bring concessions.
"I really don't view our relationship as one of quid pro quo," he told broadcaster ITV.
He ruled out US backing for any Kyoto-style deal on climate change involving legally-binding reductions of carbon emissions, while defending US efforts to tackle climate change.
"The Kyoto treaty would have wrecked our economy if I can be blunt," he said.