Since embarking in September on his speaking tour of American campuses, he has been trailed by protests. But the events have also attracted pockets of self-described anarchists clad in face masks and spoiling for a fight. Some university organizers withdrew invitations to Mr. Yiannopoulos over security concerns. At the University of California, Davis, on Jan. 13, his speech was canceled as it was set to begin after a tense standoff between protesters and police officers. A week later, on Inauguration Day, a man was shot during protests outside Mr. Yiannopoulos’s speech at the University of Washington in Seattle.