The demonstrators arrived angry, departed furious. The police had herded them into pens. Stopped them from handing out fliers. Threatened them with arrest for standing on public sidewalks. Made notes on which politicians they cheered and which ones they razzed. Meanwhile, officers from a special unit videotaped their faces, evoking for one demonstrator the unblinking eye of George Orwell's "1984." "That's Big Brother watching you," the demonstrator, Walter Liddy, said in a deposition. Mr. Liddy's complaint about police tactics, while hardly novel from a big-city protester, stands out because of his job: He is a New York City police officer....