The arrest of Jamel Jackson, 21, who was himself wounded, came as Mayor Jenny Durkan and Police Chief Carmen Best pledged an all-out effort to curb gun violence in Seattle, which has long prided itself as one of the nation's safest large cities.

"While we are better than many cities when it comes to gun violence, we are not immune to gun violence," Durkan told reporters on Thursday, a day after the shooting, which unfolded in the heart of a busy shopping district near Pike Place Market at the peak of Wednesday's evening rush hour.

An altercation outside a fast-food restaurant escalated into gun violence among three men, Chief Best recounted at the news conference.

"Guns were pulled, shots were fired, people were running and the subjects fled from each other," she said.

Scores of shoppers, commuters and pedestrians scurried for cover in the pandemonium, and victims were found spread over an area of a block and a half, officials said. One woman, about 50 years of age, was pronounced dead at the scene.

Police detectives viewing video footage recognised one of the armed men as a known gang member and convicted felon who was thus prohibited by law from carrying a gun, Best said.

He was arrested at the hospital where he and six other people had been taken for treatment of gunshot wounds and was booked on unlawful possession of a firearm, police said.

Australian Associated Press