In the afternoon, protesters trying to march toward the convention center where the gathering is being held encountered roaming squads of police officers carrying plastic shields and batons. The police fired a sound cannon that emitted shrill beeps, causing demonstrators to cover their ears and back up, then threw tear gas canisters that released clouds of white smoke and stun grenades that exploded with sharp flashes of light.
City officials said they believed it was the first time the sound cannon had been used publicly. “Other law enforcement agencies will be watching to see how it was used,” said Nate Harper, the Pittsburgh police bureau chief. “It served its purpose well.”
The protesters, who did not have a permit to march, rolled a large blue metal trash container down 37th Street. It stopped short of police vehicles and in front of a women’s clothing and shoe boutique called Pavement.
“It was scary,” said Alissa Martin, the shop’s owner. “You feel like you’re living in a war zone.”
About 20 people were arrested, and city officials said no serious injuries were reported.
On Thursday night on Forbes Avenue, hundreds of police officers carrying long batons fired smoke canisters that trailed in high arcs before landing among buildings at the University of Pittsburgh, including the one called Cathedral of Learning.
The city had locked down its business district, known as the Golden Triangle, in preparation for possible clashes. Riot fences lined the sidewalks. Police helicopters, gunboats and Humvees darted to and fro. City officials announced they had up to 1,000 jail cells ready after county officials freed up additional space last week by releasing 300 people who had been arrested on minor probation violations.
Many local residents stayed away from downtown Thursday, fearing clashes.
But the intermittent conflicts that did occur were well outside the security perimeter surrounding the G-20 meetings.
Much of the afternoon involved a cat-and-mouse game in which protesters, many in all black, evaded large forces of heavily armed police officers in the streets near Liberty Avenue.
The police repeatedly announced over loud speakers that the crowd had assembled unlawfully.
“You must leave the immediate vicinity,” the voice over the loud speaker said, adding that if the protesters did not, they would be subject to arrest and would face “the use of riot control agents” and “less lethal munitions,” which police later said were soft bean bags fired at protesters. At that point, the police fired tear gas and stun grenades.
Trevor Griffith, 21, was part of the march after driving 16 hours from Pensacola, Fla., with three fellow students from the University of West Florida.
“The fact that 20 or so individuals right now are determining economic trade policies for four to five billion people just isn’t right,” Mr. Griffith said. “That’s why we’re here.”
The turbulence downtown was in sharp contrast to smaller and less confrontational rallies and parades earlier in the day.
Melanie White, 53, from Freemont, Ohio, said she was marching to bring wider attention to the conflict between the authorities and religious leaders in Myanmar. On Thursday, she joined a rally of about 100 people led by Burmese monks in saffron robes and chanting. The group went from the north side of the city over the Sixth Street Bridge, ending up in Point State Park in downtown Pittsburgh.
“It was very important to be there,” Ms. White said, adding that her group was commemorating the second anniversary of the so-called saffron revolution in Myanmar.
“I think it is important to give voice to the Burmese problem because they are not getting their own voice at the G-20,” Ms. White said.
Just blocks away, a row of vans filled with police officers escorted several cars carrying meeting attendees past a police barrier to the David L. Lawrence Convention Center, where the meeting officially began Thursday evening with a welcoming ceremony.
Sean D. Hamill and Colin Moynihan contributed reporting.
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