Activists promised an angry welcome for leaders of the G20 economies in Pittsburgh on Thursday, with anarchist factions threatening to march on the summit venue.
Pittsburgh steels for G20 protests
"The city has bought a thousand cannisters of tear gas. That's something people are concerned about, like what to do if they're gassed," Noah Williams, a spokesman for the Pittsburgh G20 Resistance Project told AFP.

The group's unauthorized march, called in open defiance of the authorities, could result in an explosive situation, said Michael Healey, a civil rights lawyer representing 14 environmental activists who were arrested Wednesday.

"What I fear at the march is over-reaction by the authorities and over-reaction by some of the younger protesters. It's a toxic mix."

Pittsburghers gathered in a park on a spit of land between two rivers for a concert and rally on Wednesday evening were optimistic the city would not be rocked by the violence that has disrupted such gatherings in the past.

The most notorious occasion in North America was at the 1999 World Trade Organization meeting in Seattle, when anarchists in a 40,000-strong crowd fought running battles with police.

The most recent G20 summit in London also witnessed violence, and British police were criticized after footage showed an officer beating a bystander who later died.

"We're all nervous about it," said 86-year-old Jan Moravec, an environmental activist at the rally in Pittsburgh's Point State Park.

"It's all we've heard about for the past month. When we get through Friday night and we see all the world leaders heading off in their helicopters, we'll heave a sigh of relief."

Anna Lee-Fields, 35, was confident this former hard-as-nails steel town, which has undergone a stunning transformation to become a cultural hub at the forefront of green technology, would not descend into violence.

"It's Pittsburgh. So, no," she said.

"We're not such a huge city that it would get out of control," she said.

Small businesses, however, were taking few chances on the eve of the talks. Many plan to shut during the two day meeting, and workmen were boarding over shop windows in the downtown business district.

Environmental group Greenpeace kicked off the anti-G20 protests early, with activists unfurling a huge banner from a bridge in the heart of the city.

Fourteen activists were arrested after they abseiled down one of Pittsburgh's iconic river bridges to deploy a banner reading: "Danger. Climate destruction ahead," police said.

"At this point, I know that they have been charged with conspiracy, trespass, possession of instruments of crime, which is their climbing gear -- their ropes and backpacks," Healey, their lawyer, told AFP.

US secret service officers were to conduct an overnight sweep of the conference center and the area surrounding it before the summit begins on Thursday, and mounted officers and hazardous materials units were deployed nearby.

Around Point State Park, where the climate change concert was held Wednesday evening, police were erecting barbed wire.

Several groups plan to stage protests in Pittsburgh during the two-day summit, with the main event being a huge march on Friday, in which thousands are expected to take part, according to one of the organizers, Peter Shell.