I've lived in the Washington, D.C., area for the better part of the last 10 years. So I've seen my share of demonstrations, although more often than not I just try to avoid the traffic nightmares they cause. Among the various classes of protests—pro-life, anti-war, environmental, and now tea parties—the most destructive are the anti-globalization marches. So when cops clashed with anti-globalization demonstrators at the Pittsburgh G-20 summit in September, it was easy to assume that most of the altercations represented justified police responses to overzealous protesters.

But a number of disturbing photographs, videos, and witness accounts told a different story. Along with similar evidence from other recent high-stakes political events, they reveal an increasing, disquieting willingness to smother even peaceful dissent.

On the Friday afternoon before the G-20 meeting kicked into high gear, a student at the University of Pittsburgh snapped a photo showing a University of Pittsburgh police officer directing traffic at a roadblock. What's troubling is what he's wearing: camouflage military fatigues. It's difficult to discern a practical reason why a man working for an urban police department would need to wear camouflage, especially while patrolling an economic summit. He's a civilian dressed like a soldier. The symbolism is clear, and it affects the attitudes of both the cops wearing the clothes and the people they're policing.

The campus cop wasn't alone. Members of police departments from across the country came to Pittsburgh to help during the summit, most of them dressed in paramilitary garb. In one widely circulated video, several officers dressed entirely in camouflage emerge from an unmarked car, apprehend a young backpack-wearing protester, stuff him into the car, and drive off. The sequence evoked the "disappearances" associated with Latin American dictatorships or Soviet Bloc countries. When Matt Drudge linked to the video, he described the officers in it as members of the military. They weren't, but it's easy to understand how someone might make that mistake.

In another video, members of a police unit from Chicago who took vacation time to work at the summit prop up a handcuffed protester and gather behind him. Another officer then snaps what appears to be a trophy photo. Two men in faraway Queens were arrested for posting the locations of riot police on Twitter, as though they were revealing the location of troops on a battlefield. Another video shows dozens of police in full body armor confronting and eventually macing onlookers (who weren't even protesters) in the neighborhood of Oakland, far from the site of the summit, as a recorded voice orders any and all to disperse. Students at the University of Pittsburgh claim cops fired tear gas canisters into dorm rooms, used sound cannons, and shot bean bags and rubber bullets.

The most egregious actions took place on September 25, when police began ordering students who were in public spaces to disperse despite the fact that they had broken no laws. Those who moved too slowly, even from public spaces on their own campus or in front of their dorms, were arrested. A university spokesman said the aim was to break up crowds that "had the potential of disrupting normal activities." Apparently a group of people needn't actually break any laws to be put in jail. They must only possess the "potential" to do so, at which point not moving quickly enough for the cops' liking could result in an arrest. That standard is a license for the police to arrest anyone anywhere in the city at any time, regardless of whether they've done anything wrong. In all, 190 people were arrested during the summit, including at least two journalists.

It can't be easy to both keep order and protect civil liberties at such events. But that doesn't mean police and city officials shouldn't be expected to try. Yes, some protesters damaged some property at the G-20 summit, although there wasn't much of that this time around. But the presence of a few unruly demonstrators doesn't give the police carte blanche to crack down on every young person in the general vicinity, nor should it give the city free rein to suppress all public protest. It's unfortunate that when the global press and the leaders of the world's 20 largest economies came to Pittsburgh, the images that emerged were not of a society that values free expression and constitutional rights but of one willing to grant police powers normally seen in authoritarian states.

This projection of overwhelming force at big events is becoming more common. At last year's Republican National Convention in Minneapolis, police conducted peremptory raids on the homes of protesters before the convention began. In all, 672 people were jailed, including at least 39 journalists. According to the Minneapolis Star-Tribune, 442 of those 672 later had their charges either dropped or dismissed.

Four years before that, more than 1,800 people were arrested at the previous Republican National Convention in New York City. Ninety percent were never charged with a crime. One notorious photo from the 2008 Democratic National Convention in Denver shows a small mass of protesters, zoned far off from where any delegates or media representatives could hear them, surrounded by two walls of riot police who outnumbered them at least 2 to 1. Denver's police union later issued a commemorative T-shirt of the event emblazoned with an illustration of a menacing cop wielding a baton and the slogan, "We get up early to beat the crowds."

The trend may have started at the 1999 World Trade Organization summit in Seattle, which saw both actual rioting and police overkill. Mayor Paul Schell not only declared a state of emergency, imposed a curfew, and designated swaths of the city "no-protest" zones; he actually banned civilian possession of gas masks. Police then gassed entire city blocks. The victims included many owners of the stores the police were ostensibly protecting from looters. Assistant Police Chief Ed Joiner, who was in charge of security for the event, would later tell reporters that future summits should be held only in destinations with military governments.

These are precisely the kinds of events where free speech and the freedom to protest need protection the most: when influential figures make high-level decisions with far-reaching consequences. Instead, we see the opposite. The higher the event's profile, the more powerful the players involved, and the more important the decisions being made, the more determined police and politicians are to make sure dissent is kept as far away from the VIPs as possible. Or silenced entirely.

Radley Balko (rbalko@reason.com) is a senior editor at reason.