OAKLAND, Calif. — For supporters of the Occupy Wall Street movement, whose diffuse anger has been a defining and sometimes distracting characteristic, the wounding of an Iraq war veteran here has provided a powerful central rallying point.

The veteran, Scott Olsen, 24, was critically injured on Tuesday night when he was hit in the head with a projectile thrown or shot by law enforcement officers combating protesters trying to re-enter a downtown plaza that had been cleared of an encampment earlier in the day. Mr. Olsen, who served two tours of duty in Iraq as a Marine, suffered a fractured skull.

And while Mr. Olsen’s condition has since improved, his injury — and the oddity of a Marine who faced enemy fire only to be attacked at home — has prompted an outpouring of sympathy, as well as calls for solidarity among the scores of Occupy encampments around the nation. On Thursday night, camps in several major cities — including New York, Chicago and Philadelphia — were expected to participate in a vigil for Mr. Olsen, according to Iraq Veterans Against the War, of which he is a member.

“I think people would have been outraged even had this been a civilian,” said Jose Vasquez, the group’s executive director, “but the fact that he survived two tours of duty and then to have this happen to him, people are really upset about that.”