Police fired canisters of pepper spray and smoke and rubber bullets at marchers protesting the Group of 20 summit Thursday after anarchists responded to calls to disperse by rolling trash bins, throwing rocks and breaking windows.
Pittsburgh Police Chief Nate Harper said 17 to 19 protesters were arrested, and Mayor Luke Ravenstahl said swift police decisions resulted in minimal property damage. Officials said there were no reports of injuries.
The afternoon march turned chaotic at just about the time that President Barack Obama and first lady Michelle Obama arrived for a meeting with leaders of the world's major economies.
The clashes began after several hundred protesters, many advocating against capitalism, tried to march from an outlying neighborhood toward the convention center where the summit is being held.
The protesters clogged streets, banged on drums and chanted "Ain't no power like the power of the people, 'cause the power of the people don't stop."
The marchers included small groups of self-described anarchists, some wearing dark clothes and bandanas and carrying black flags. Others wore helmets and safety goggles.
One banner read, "No borders, no banks," another, "No hope in capitalism." A few minutes into the march, protesters unfurled a large banner reading "NO BAILOUT NO CAPITALISM" with an encircled "A," a recognized sign of anarchists.
The marchers did not have a permit and, after a few blocks, police declared it an unlawful assembly. They played an announcement over a loudspeaker ordering people to leave and then police in riot gear moved in to break it up. Authorities also used a crowd-control device that emits a deafening siren-like noise, making it uncomfortable for protesters to remain in the streets.
Protesters split into smaller groups. Some rolled large metal trash bins toward police, and a man in a black hooded sweat shirt threw rocks at a police car, breaking the front windshield. Protesters broke 10 windows in a few businesses, including a bank branch, a Boston Market restaurant and a BMW dealership, police said.
Officers fired canisters of pepper spray and smoke at the protesters, set off a flash-bang grenade and fired rubber bullets. Some of those exposed to the pepper spray coughed and complained that their eyes were watering and stinging.
About an hour after the clashes started, the police and protesters were at a standoff. Police sealed off main thoroughfares to downtown.
Twenty-one-year-old Stephon Boatwright, of Syracuse, New York, wore a mask of English anarchist Guy Fawkes and yelled at a line of riot police. He then sat cross-legged near the officers, telling them to let the protesters through and to join their cause.
"You're actively suppressing us. I know you want to move," Boatwright yelled, to applause from the protesters gathered around him.
Protesters complained that the march had been peaceful and that police were trampling on their right to assemble.
"We were barely even protesting," said T.J. Amick, 22, of Pittsburgh. "Then all of a sudden, they come up and tell us we're gathered illegally and start using force, start banging their shields, start telling us we're going to be arrested and tear gassed. ... We haven't broken any laws."
Bret Hatch, 26, of Green Bay, Wisconsin, was carrying an American flag and a "Don't Tread on Me" flag.
"This is ridiculous. We have constitutional rights to free speech," he said.
The National Lawyers Guild, a liberal legal-aid group, said one of its observers, a second year law student, was among those arrested. Its representatives were stationed among the protesters, wearing green hats.
"I think he was totally acting according to the law. I don't think he was provoking anyone at all," said Joel Kupferman, a member of the guild. "It's really upsetting because he's here to serve, to make sure everyone else can be protected. ... It's a sign that they are out of control."
The march had begun at a city park, where an activist from New York City, dressed in a white suit with a preacher's collar, started it off with a speech through a bullhorn.
"They are not operating on Earth time. ... They are accommodating the devil," he said. "To love democracy and to love the earth is to be a radical now."
The activist, Billy Talen, travels the country preaching against consumerism. He initially identified himself as "the Rev. Billy from the Church of Life After Shopping."
Such street demonstrations have become the norm at world economic gatherings, including a G-20 meeting in London in April. The protesters here appeared to number fewer than a 1,000, a fraction of the 50,000 that took to the streets of Seattle a decade ago at a World Trade Organization event.
Later Thursday, hundreds of protesters, including a handful of anarchists, massed near the Phipps Conservatory and Botanical Garden where the G-20 summit was beginning with a welcome ceremony.
"Tell me what a police state looks like. This is what a police state looks like!" the protesters chanted as several hundred riot police blocked them from getting any closer.
Police fired smoke canisters on a crowd at night in the city's Oakland section, home to the University of Pittsburgh, after calling for people to disperse, calling it an illegal assembly.
Police had let the crowd, a mix of protesters and students, remain for several hours before issuing the dispersal order. Police appeared to arrest a few people.
Dignitaries arrived in waves throughout the day, entering a city under heavy security. Police and National Guard troops guarded many downtown intersections, and a maze of tall metal fences and concrete barriers shunted cars and pedestrians.
The G-20 ends late Friday afternoon after a day of meetings at the David L. Lawrence Convention Center.