The protesters, many of them wearing gas masks, stormed into the security zone and succeded in pulling down a 160-metre stretch of a perimeter fence.
The protesters, many of them wearing gas masks, stormed into the security zone and succeded in pulling down a 160-metre stretch of a perimeter fence.
They also hurled stones and bottles at riot police, who responded with tear gas and baton charges.
Several dozen riot police formed a line and marched on the demonstrators, forcing them to retreat.
Reports said two officers were injured and demonstrators had smashed window of a nearby petrol station.
Diverse range
The vast bulk of demonstrators argue that free trade, one of the goals of the three-day summit of leaders of 34 countries in the Americas, hurts the poor and the environment.
The demonstrators represent a diverse range of activists - human rights organisations, environmental groups and organised labour.
Protests are also planned far from Quebec City, with marches or blockades threatened in other Canadian and US cities. Cyberprotests could also occur.
Quebec authorities have already arrested some demonstrators who they say were planning on violent protests.
'Free hemisphere'
President Bush, who will meet Latin American leaders individually later on Friday, has launched a charm offensive to try and persuade his fellow heads of government of the advantages of free trade.
He told reporters that his goal was to create an "entire hemisphere that is both prosperous and free."
"It is a great task to make the Americas the Land of Opportunity" he said.
"Our goal in Quebec is to build a hemisphere of liberty. We must approach this goal in a spirit of civility, mutual respect and appreciation for our shared values," Mr Bush added.
Mr Bush made it clear that as well as discussing the drug problem, promoting his free trade plan was his top priority.
Resistance
Mr Bush faces the most resistance from Brazil, the largest economy in South America, which would like to see US concessions on agricultural subsidies.
The role of Mexico is also in doubt. Currently the only Latin American country to enjoy unrestricted access to US markets, it is loathe to give up that privilege.
The Free Trade Area of the Americas (FTAA) would encompass nearly all the countries of the Americas with a combined population of 800 million who produce goods and services totalling $11.4 trillion - bigger than the European Union.
The plan, if approved by 2005, would eliminate or make very low tariffs on goods moving between countries. It would also streamline customs regulations, eliminate quotas and subsidies and remove other impediments to trade.