Police threw canisters of pepper spray and smoke at anarchists protesting the Group of 20 summit Thursday after the marchers responded to calls to disperse by rolling trash bins and throwing rocks.
The march turned chaotic at just about the same time that President Barack Obama and first lady Michelle 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.
Police in riot gear stood guard near the protesters, who banged on drums and chanted "Ain't no power like the power of the people, 'cause the power of the people don't stop."
Efforts to disperse crowd
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 moved in to break it up.
Protesters split into smaller groups. Some rolled trash bins toward police, and a man in a black hooded sweat shirt threw rocks at a police car, breaking the front windshield. Some protesters used pallets and corrugated steel to block a road. Police said the windows at one bank branch were broken.
Officers fired pepper spray and smoke at the protesters. Some of those exposed to the pepper spray were coughing, complaining of eyes watering and stinging.
Some of the protesters were seen ducking into alleyways to change out of their all-black clothing.
The hundreds of marchers had included small groups of self-described anarchists, some wearing dark clothes and bandanas and carrying black flags and others wearing helmets and safety goggles. Some held a banner that read, "No borders, no thanks." Another banner read, "No hope in capitalism."
Protesters unfurled a large banner reading "NO BAILOUT NO CAPITALISM" with an encircled "A," a recognized sign of anarchists.
'To be a radical'
An activist from New York City, dressed in a white suit with a preacher's collar, kicked off the march 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 begins Thursday evening with a welcome ceremony 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.