David Usborne: Pittsburgh gears up to stop protesters crashing party

Protesters clashed with riot police on the streets of Pittsburgh last night as world leaders gathered in the city for the G20 summit. There were reports that pepper gas and rubber bullets were used to disperse pockets of demonstrators who had tried, but failed, to reach the convention centre.

Thousands of extra police were on stand-by, helicopters hovered, sirens sounded, and businesses, some with boarded up windows, stood abandoned for the two-day duration. As the Chinese leader Hu Jintao arrived at the Westin Hotel near the conference centre, onlookers were barricaded behind high steel fences and all access to the area was sealed off by empty lorry trailers. Large contingents of riot police and some on horses roamed nearby avenues giving the impression of a city under martial law. A seemingly exaggerated number of police with batons pointed forward and chanting "move, move" broke up a small group of startled demonstrators protesting against repression in Ethiopia.

The residents of Pittsburgh greeted the arrival yesterday of the G20 circus as they would a freak autumn snow storm. For two days, their city has become almost unrecognisable – transformed in a way that may be exhilarating but also deeply inconvenient and possibly destructive. They are meant to be flattered. The White House chose Pittsburgh for the summit because of its success in shaking off the rust of its collapsed steel industry and re-inventing itself with a vibrant knowledge-based economy. Unemployment here is now several points lower than the national rate. It is why last night the G20 leaders, with Barack Obama as host, dined in the shining Phipps Conservatory, a botanical wonderland under glass that boasts near self-sufficiency in electricity and water. The main talks this morning take place downtown under the ski-jump roof of the newly-built David Lawrence Convention Centre, billed as the first such facility to be green certified in the world.

Michelle Obama, moreover, will be doing her best to show the area off to the other very important spouses in town, beginning with a separate dinner yesterday at a farm just outside the city, owned by Teresa Heinz, the wife of Senator John Kerry.

Yet the city did not feel like a place celebrating. Rather it seemed almost to be cowering under the threat of mass protests. Downtown – squeezed into a compact spit of land at the confluence of the Allegheny and Monongahela rivers – was in virtual lockdown. "The G20 is in the house, throwing a party," declared the Resistance Project, one group co-ordinating the protest action. "Let's crash it". Just as the G20 agenda seemed hard to summarise, so too were the motivations of the protesters. The anger of marchers, most of whom had been corralled into controlled areas by police, had to do variously with climate change, bankers' greed, globalisation or just government generally. The fear for cities asked to host summits like this is a repeat of what Seattle experienced at its world trade summit in 1999 which ended with tear gas, millions of dollars in damage to buildings and scores of arrests.

The Resistance Project was urging marchers to target a list of private businesses it says are bent on global hegemony. Obvious among them were branches of McDonalds and Starbucks. But singled out also were some independent businesses like John Byrnes' 24-hour gym. He wonders why: "I'm just as curious as anybody why they're protesting [against] my gym."