At one stage the festival atmosphere risked being overshadowed by gangs dressed in black clothing and balaclava hats, who rampaged through downtown smashing windows, spray-painting slogans, attacking cars and squirting oil and paint on photographers' camera lenses.
Phalanxes of riot police fired pepper spray, tear gas and rubber bullets at the crowds at some intersections. Several protesters were temporarily blinded and two were seen staggering away with blood on their faces. But the violence largely subsided as a vast flow of protesters, co-ordinated by US trade unionists, joined the more radical groups which had blocked the area around the Convention Center.
A patchwork of environmentalists, animal rights activists, students and trade unionists dressed as sea-turtles and undertakers sang, danced, chanted anti-capitalist slogans and formed a great snaking line stretching two miles from Memorial Stadium through downtown and back out again.
The protests threw the WTO meeting into chaos, as delegates found themselves stranded and some where physically attacked. The American delegation was completely surrounded in its hotel. The opening ceremony was delayed six hours, and then disrupted by three protesters who reached the event despite heavy security. The Sheraton Hotel, where the EU delegation stayed, was surrounded by chanting protesters and at one stage tear gas drifted through its elegant lobby and meeting rooms choking delegates, including Glenys Kinnock.
Many complained that little serious work had been achieved, and some openly questioned whether the event could go ahead if the protests continued today. But Mike Moore, Director General of the WTO, held a defiant press conference where he promised: "This conference will be a success.
"I will not accept that somehow this is not a democratic organisation," he said defiantly. "I will defend the right of our ministers to sit around a table to settle their differences."
President Bill Clinton is due to arrive in Seattle today, but with protesters still milling around the streets it is hard to see how his motorcade could get into town, much less that he would be able to walk around the streets safely.
At the start of the morning, with drums throbbing, whistles blowing and heavy rock and roll music blaring through the streets, the first wave of radical demonstrators pirouetted and danced towards the venue. "Shut it down!" they chanted. "We ain't gonna leave till you get out of Seattle. Go home!"
Entrances to the conference sites were blocked by spoof police cordon tape reading "Unseen Crimes" - a reference to allegation that the WTO works secretly to promote the interests of multinational corporations at the expense of labour standards, the environment and human rights. "Block the streets! They can't move all of us!" shouted the organisers.
This first protest was not authorised by the police, but officials initially seemed to be relaxed about it converging on downtown as long as it remained peaceful and did not block traffic getting on and off Interstate 5, the main freeway running through the city.
Seattle's mayor, Paul Schell, arrived at the besieged Paramount Theater, venue for the opening ceremony that never happened, and said simply: "You can be firm with your message, but be gentle with your city." A group of protesters jumped on to the roofs of a ring of city buses set up by police as a barricade outside the theatre. But they climbed back down again after riot police threatened to remove them.
The most serious violence broke out a couple of hours later, beginning with confrontations at street intersections and then degenerating as a small group hurled Molotov cocktails, smashed storefronts and sprayed graffiti declaring: "Corporate greed sux." The violence then subsided as the trade union demonstration swelled through the afternoon.
It was the largest public demonstration in Seattle since the Vietnam War. With a whole cluster of issues at stake, from genetically modified foods to the rights of Zapatista rebels in Mexico, the slogans on display represented a kind of alternative globalism to that being pushed by the WTO itself. "Fair trade, not free trade," said the banners, interspersed with whale balloons, large animal skeletons representing endangered species and big photographs of corporate leaders adorned with hate slogans.
Some protesters urged police and WTO delegates to join them. Others resorted to teasing slogans broadcast over a loudspeaker system: "WTO - You've been very naughty delegates. Go to your room!"
The protests began before dawn. Groups of demonstrators, typically sporting body piercing and individual-sized coffee thermoses, emerged with their banners all over Capitol Hill, a student area above downtown. "Whose streets? Our streets!" they chanted. Several banners played with the WTO acronym, suggesting "World Takeover Organisation" and "Way Too Orwellian" as alternatives. An older man in an Uncle Sam outfit had dollar bills pinned to his coat tails and symbolically swung at a football-sized globe with a baseball bat.
Speeches broadcast over the loudspeakers denounced the capitalist system and accused the WTO of putting profits before people and the planet.
Seattle has a long history of civil protest stretching back to a general strike in 1919, and yesterday it was all but shut down. The port closed and several groups of workers, including taxi drivers, went on strike for the day. Many joined the union demonstration, which also featured groups from Canada, France and Mexico. Two women marched topless with the slogan "My body is not a commodity" written on their backs.
The proceedings were cheered on by heroic figures from the 1960s, including Ken Kesey, organiser of the Merry Pranksters and author of One Flew Over the Cuckoo's Nest, and Tom Hayden, one of the Chicago Eight who demonstrated against the Vietnam War during the Democratic Party convention in 1968. "In 1968, we had one or two issues, but this is about everything," said Mr Hayden, 60, now a California state senator.
One Microsoft employee, trying in vain to get to her office, muttered: "This is crazy. Why can't they put all this in some field in Montana?"