City leaders in Portland, Ore., are considering making it illegal for protesters to wear masks in an attempt to address violent clashes between left-wing and right-wing activists, the latest of which occurred a few weeks ago.

Police Chief Danielle Outlaw first called for an antimask law after dueling protests on June 29, where a conservative writer said he was assaulted by members of the left-wing group Antifa, who frequently wear masks. Demonstrators from Antifa, short for antifascist, were among those countering a march...