Police have used rubber bullets, tear gas and water cannons against anti-globalisation protesters in Swiss and French cities near Evian where the Group of Eight (G8) summit is being held.
Authorities spent hours battling with protesters
A 15-kilometre (10-mile) exclusion zone has been placed around the summit venue itself to prevent protesters from getting close to the politicians and delegates.
In the Swiss city of Geneva authorities spent more than nine hours battling with demonstrators as they rampaged through the city centre.
Shop windows were smashed and stores looted, leaving the city streets awash with broken glass and choking fumes from tear gas canisters.
After protesters began to hurl rocks and petrol bombs, the German police were brought in for reinforcements, storming the front line to scatter the rioters and chasing ringleaders all over the city, the BBC's Emma Jane Kirby in Geneva said.
In Lausanne demonstrators wearing black face masks blocked roads with burning barricades and attacked the hotel area where some summit delegates were staying before being driven away by riot police with tear gas.
Several demonstrators were injured, one seriously, reports said.
The protests came as many world leaders, including US President George W Bush, gathered in Evian for the summit of the world's leading industrialised nations.
Bush and Chirac: Brief handshake and taut smiles
Protesters say the summit will achieve little in terms of addressing the needs of the world's poor.
At the summit, the diplomatic rift caused by the US-led war in Iraq was expected to overshadow proceedings.
France's President Jacques Chirac, the leading opponent of the war, greeted Mr Bush with a handshake and a smile on his arrival, in what was their first meeting since the war.
The French president also sought to play down differences with his American counterpart.
He praised Mr Bush's recent announcement of a major US financial contribution to the battle against Aids in the developing world, and called on the European Union to make a parallel effort.
For their part, the US is said to be in no mood to forgive so-called diplomatic "wrecking tactics" employed by France and another G8 member, Germany, that prevented United Nations Security Council backing for the war.
However, Mr Bush appears to have healed the divisions with Russian President Vladimir Putin - another leading opponent of the war - during their talks in St Petersburg earlier on Sunday.
Action against terrorism
Other matters on the agenda at the summit include another attempt to improve life for people in the developing world, with debt relief, access to cheap medicines, and agricultural subsidies that favour rich countries on the agenda.
President Chirac also invited 12 countries from Asia, Latin America, Africa and the Arab world for talks on development issues during the first day of the summit.
President Chirac said it was necessary for the G8 to hear the views of others, and for the others to feel that they were heard and respected.
Mr Bush will press for further action against terrorism and against countries accused of developing weapons of mass destruction.
In particular, he is seeking international authority to seize such weapons, or their components, when they are being carried by ship or plane. There are suggestions that the French are prepared to sign up to this.
Months of planning
The local authorities had promised they were well prepared for the G8 protests, saying they expected up to a 100,000 demonstrators.
In the event, our correspondent says they were overrun by just a few hundred troublemakers - and with millions of dollars of damage done to their homes and businesses, the people of Geneva will want to know how that happened.
The authorities had hoped to avoid a repetition of the violence at the G8 summit in Genoa, Italy, two years ago when an Italian protester was shot dead by police.
The summit ends on Tuesday.
The G8 countries are: the United States, Japan, Germany, UK, France, Italy, Canada and Russia.