Seattle Mayor Mike McGinn warned Friday that scheduled protests on May 1 could turn violent.
McGinn said Seattle officials "have evidence" that people may be traveling to the city "with the intention of using the public demonstrations as an opportunity to commit violence, damage property and disrupt peaceful free speech activity."
He also noted "a significant increase in graffiti and posters alluding to violence around the May 1 events."
May 1 is May Day, celebrated around the world as a tribute to workers. Already planned events in Seattle include a march for immigrant rights, an Occupy Seattle "general strike," and rallies scheduled at Westlake Park in downtown.
McGinn said websites have been found that "described trainings in how to conceal weapons beneath signs and banners, and how to target police officers on horses" -- such as the violence at the Occupy the Port protest last December.