Defying riot police who repeatedly fired rubber bullets and tear gas at them, the anti-globalisation foot soldiers created a siege-like atmosphere by massing at several points of the perimeter fence erected to protect the 34 leaders at the Summit of the Americas.
Eye-stinging tear gas floated through the sealed-off zone in the historic city and entered venue building vents, reminding the presidents and prime ministers that violence marring major summits has become as predictable as their closing statements.
Hardened by battles that derailed such events as the 1999 world trade talks in Seattle, masked militants dressed in black fought into the early hours of Saturday with helmeted police carrying shields.
They threw burning rolls of toilet paper inside the fence as battles flared sporadically less than 800 metres from where leaders, including US President George W, were staying.
For about an hour on Friday, they breached the 3 metre-high chain link fence embedded in concrete, forcing a 90-minute delay to the summit's opening ceremony, where the key issue was the proposed creation of the world's largest trade area.
Thousands of demonstrators, ranging from avowed peaceniks to hard-core anarchists, have crowded into the French Canadian city to protest a proposed pact to create by the end of 2005 a free trade area from Alaska to Patagonia.
In contrast with the protesters, who threw rocks, bottles and hockey pucks, hippy-like demonstrators ignored the violence and, remaining at the front-line, danced, played drums and gently batted a large beach-ball over the heads of militants.
Free trade is fair trade?
Police said they arrested 28 people and five officers were injured, mainly in clashes at the metal fence that snakes for 6km around summit venues and has been dubbed the "wall of shame" by protesters.
The prospect of the free trade zone embracing 800 million people has galvanised a generation of activists, as the Vietnam War and nuclear arms did previously. They say it is designed to benefit big corporations, not Latin America's poor.
While the Friday-to-Sunday summit will officially discuss trade for only a scheduled 30 minutes, the protesters have shoved the issue to the top of the popular agenda and prompted leaders to defend their proposal.
Bush, who has declared himself an aggressive free trader, will address the summit's first working session on Saturday. After Friday's violence, which upset his timetable for meetings with regional leaders, he said the protesters were wrong.
"Trade not only helps spread prosperity, but trade helps spread freedom ...," he told reporters. "We need trade."
Summit host, Canadian Prime Minister Jean Chretien, said in his opening speech: "I welcome those who have come to Quebec City to make known their views on how best to advance the social and economic interest of our fellow citizens. But violence and provocation is unacceptable in a democracy."