Video footage showd the moment police in Seattle deployed pepper spray and 'blast balls' to disperse demonstrators during a May Day rally in the north-western US city on 1 May.
Violent clashes broke out between the Seattle Police Department and black-clad, anti-capitalist protesters, who were out supporting worker and immigration rights. Authorities say rocks, flares, bricks and Molotov cocktails were thrown at some of the officers.
The footage, from Twitter user Sam Levine, shows police urging people to move back as explosions from the blast balls can be heard and smoke appears in the crowd.
Seattle Police Department has described 'blast balls' in the past as a "firework-like device which gives off a loud bang, a flash & a modest dose of pepper".
At least nine people were arrested before the protesters were contained within a Costco parking lot. Authorities say that five officers suffered minor injuries.