Police fired canisters of pepper spray and smoke at marchers protesting the Group of 20 summit Thursday after anarchists responded to calls to disperse by rolling trash bins and throwing rocks.
The 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 hundreds of protesters, many advocating against capitalism, tried to march from an outlying neighborhood toward the convention center where the summit is being held.
The ACLU estimates that as many as 1,000 people are participating, although the group that assembled was much smaller than had been anticipated reports CBS Station KDKA in Pittsburgh.
The protesters 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 thanks," 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 telling people to leave or face arrest and then police in riot gear moved in to break it up.
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 windows in a few businesses, including a bank branch and a Boston Market restaurant.
Officers fired pepper spray and smoke at the protesters. Some of those exposed to the pepper spray coughed and complained that their eyes were watering and stinging.
The Secret Service told KDKA in Pittsburgh that the police used OC vapor and smoke, not tear gas.
Police were planning a news conference to discuss their response. Officers were seen taking away a handful of protesters in cuffs.
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, N.Y., 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, Wis., 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 Lawyer's 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."
The G-20 summit was beginning Thursday evening with a welcome ceremony at the Phipps Conservatory and Botanical Garden and ends late Friday afternoon after a day of meetings at the David L. Lawrence Convention Center.
Dignitaries were arriving in waves and were heading to 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.
Hundreds of police in riot gear were seen massing at Phipps, but only a handful of demonstrators were there.
Local video from CBS Station KDKA