A US jury has found Seattle authorities guilty of violating the civil rights of demonstrators detained during the 1999 World Trade Organization meeting.
The 1999 anti-globalisation clashes were dubbed the Battle of Seattle
A federal jury found the city liable for the unreasonable search and arrest of about 170 protesters but found no breach of their right to free speech.
The city, which could face a bill for millions in compensation, is to appeal.
Nearly 50,000 anti-globalisation protesters clashed with police in 1999 - now known as the Battle of Seattle.
"The key point, the lesson learned, is you cannot arrest peaceful protesters here in Seattle or anywhere else in the country," said Kenneth Hankin, the lead plaintiff in the case, in reaction to the jury's decision.
Lawyers for the city, which suffered millions of dollars in damage after violence broke out during the WTO meeting, say they will mount a challenge.
The city has already paid about $800,000 (£410,000) in more than a dozen WTO lawsuits and claims.
The next stage in the case is a trial to determine how much Seattle must pay out in damages - if the amounts are similar to previous cases, the final bill could run to millions, correspondents say.
Seattle erupted into chaos in December 1999 when some 50,000 anti-globalisation demonstrators overwhelmed police and closed down parts of the WTO meeting.
The city's mayor declared parts of the city no-protest zones - but opened the areas to WTO delegates, shoppers and business owners. Police arrested anyone who protested within the zones.
Prior to the trial, a US district judge ruled that police had made the mass arrests without probable cause.
Jurors had to determine whether the city could be held financially responsible for the false arrests and whether the city had a policy to target protesters for their anti-WTO views.
Defence lawyers said Seattle had a legitimate reason to detain the demonstrators.
The plaintiffs, they argued, had brought no evidence they were arrested because of their political views.
But Mike Withey, a lawyer for the protesters, said the jury's verdict was "a victory for the constitutional rights we all enjoy".