Hong Kong, Aug. 26 -- Hong Kong police drew their guns and fired a warning shot after protesters attacked officers with sticks and rods, and brought out water cannon trucks for the first time, in an escalation in the summerlong protests that have shaken the city's government and residents.

Sunday's main showdown took place on a major drag in the outlying Tsuen Wan district following a protest march that ended in a nearby park. While a large crowd rallied in the park, a group of hard-line protesters took over a main street, strewing bamboo poles on the pavement and lining up orange and white traffic barriers and cones to obstruct police.

After hoisting warning flags, police used tear gas to try to disperse the crowd. Protesters responded...