On Monday, three peace protesters were arrested for third-degree assault. This is not unusual. Last month, a peace protester was arrested for assault against suspected Republicans. In 2002, a march of 1500 peace protesters in Belgium turned violent and 29 were arrested. In 1968, 200 were arrested in a violent peace protest in London. Historically, if peace protesters favor peace, it is only as an end not as a means.
UPDATE: James Taranto has two more current examples (scroll to "They love to hate hate") of peace protesters getting violent.