The demonstrations aimed at disrupting the start of the World Trade Organisation talks had started in carnival mood both in the US and in London, but were hijacked by violent protesters.
In Seattle, riot police fired tear gas, pepper spray and rubber bullets at tens of thousands of protesters blockading the WTO meeting while roving gangs of anarchists smashed windows, overturned newspaper stands and attacked cars.
With night falling and demonstrators refusing to disperse, the city declared a state of civil emergency and said it would impose a curfew from 7pm until sunrise. Police would be free to use gas and other means to keep the streets clear during that time.
Some 200 National Guards were ordered into the city to aid riot police, who moved in late last night in force to break up the massive demonstrations. Firing volley after volley of tear gas, police and armoured vehicles swept the streets while the city's mayor, Paul Schell and the police chief, Norm Stamper, held a stormy press conference to the sounds of wailing sirens, protesters chants and the discharges of riot police weapons.
Mayor Schell said: The last thing I ever wanted to do was be the mayor of a city that called in the National Guard. It makes me sick. But this city needs to be protected and secured."
The events marked a huge upsurge in the US against globalisation, focused on a summit of the World Trade Organisation which now looks to be seriously in doubt.
Hundreds of riot troops marched past smacking their batons against their black body armour as tear gas grenades could be heard exploding across the city. President Bill Clinton is due to arrive today, but with every likelihood that the protests will intensify the chances are rising that the summit will face severe problems in reaching an agreement.
The unrest was mirrored in London, where hundreds of demonstrators took over a previously peaceful protest in support of the parallel demonstrations in Seattle.
There were running battles with riot police at Euston station, where several hundred demonstrators began attacking police and destroying property. At one point, rioters overturned and set fire to an empty police van, to roars of approval from bystanders.
Some 38 arrests were made and seven people, including a police officer, were injured.
The clashes cast a shadow over one of the biggest demonstrations in the US since the Vietnam War. The day began as a peaceful attempt to keep thousands of international delegates away from their meetings at the Convention Center and to block off the opening ceremony in the city centre Paramount Theater.
The protests, billed as a giant carnival against corporate capitalism, successfully held up the WTO meeting for six hours, and some officials said a second day of demonstrations might force them to close it down.
Tens of thousands of young people converged on the streets around the Convention Center, forming human barriers three or four people deep. They were later joined by a massive demonstration organised by trade unionists.
But after three hours of untroubled protests, volleys of tear gas were fired, first at an intersection near the city's main highway, and then at more intersections where the anarchist element - not thought to number more than a few dozen - provoked showdowns by overturning rubbish skips, rolling garbage cans down hills and hurling petrol bombs.
Later, they attacked a McDonald's restaurant, a Starbucks cafe, two banks and a Gap clothing store - all viewed as proponents of the so-called "corporate agenda".
Further reports, page 3