John Nilles, 74, a Marine from Medford who served in Vietnam and is a member of the group, Veterans for Peace, said he was knocked down during the arrests.

He believes he did not get arrested in the chaos because he got knocked down, and banged up his knee.

“I have absolutely no use for police anymore,” he said. “I don’t know what I’m going to do.”

He talked about how it reminded him of the 1960s. “This is not the world I intended to come back to,” he said in reference to Vietnam.

Occupy Boston, in a statement Monday night, said it answered the police warning by issuing a call “for any and all people to join the occupation as soon as possible.”

“From the beginning, occupiers have worked tirelessly to maintain a positive working relationship with city officials. Today’s threats by the Boston Police Department represent a sudden shift away from that dialogue,” the statement said.