Police declare riot in Portland after May Day protesters break windows

Demonstrations in Portland to commemorate May Day Saturday turned violent overnight, with officials declaring a riot and arresting six people, one with a knife.

Three separate Starbucks stores and a Hilton hotel in the Oregon city were marred by graffiti, and one man was charged with menacing cops with a knife, Portland police said on Twitter.

May 1, or May Day, is celebrated in many nations as International Workers Day or Labor Day.