When the G-20 summit convenes in Pittsburgh four months from now the agenda is likely to reach beyond the guarded halls of the David Lawrence Convention Center onto city streets, where the effects of economic policies are felt in the lives of ordinary people from Hong Kong to Homestead.

Ostensibly, the G-20 will assemble to thrash out an economic agenda for the world. Comprising states that account for more than 85 percent of the planet's gross domestic product, the G-20's policies can have a universal effect. That doesn't always translate into universal approval.

More likely, it will be the stuff of political street theater.

"I think right now it stands for chaos and it stands for economic destruction," said Leo Gerard, president of the United Steelworkers of America, the region's most prominent industrial union and a harsh critic of what he sees as a widening economic imbalance growing out of the global policies put forward by many of the G-20's leaders.

"The G-20, and prior to that the G-8, whatever the G was, bought into what was euphemistically called the Washington consensus of deregulation, laissez-faire economics and I think that was the foundation of the economic collapse," Mr. Gerard said.

That collapse has become the catalyst for sometimes raucous action on the pavement outside the gathering places of the powerful.

Mr. Gerard, a Canadian, was among the labor figures who joined in protests in London earlier this year in the run-up to the G-20 meeting there.

While his segment of the event was peaceful, other street demonstrations degenerated into violent confrontations in which one man died and police and protesters traded blows in the British capital.

What lies in store for Pittsburgh is unclear.

Partly, the hour is early, especially by the often fluid standards of the groups who might have a stake in bringing their grievances to the sidewalk. Too, there is the question of which local groups might find cause to march -- labor unions, anti-war and social justice groups such as the Thomas Merton Center and the Pittsburgh Organizing Committee have already signaled interest.

Mr. Gerard would like to see representatives of organized labor invited inside the hall to address the G-20.

Locally, other groups are examining how to put their messages on the city's sidewalks.

"You need to do an incredible amount of public education in advance," said Mel Packer, a member of the Merton Center who has long been active in protest work.

Mike Healey, an official with the left-leaning National Lawyers Guild, said his inbox has already begun filling with e-mails from various groups seeking counsel on how to go about putting together demonstrations around the G-20.

Mr. Healey said he wonders what measures authorities plan to take to keep protesters out of earshot of the convention hall -- the major worry of lawyers like him and Vic Walczak, executive director of the American Civil Liberties Union of Pittsburgh. Too, Mr. Walczak worries that Pittsburgh police will use hardline tactics that could inflame more militant groups coming in from outside.

"If they do that with this group at G-20, it's going to be a mess, because the people who come in here for that are going to be twice as aggressive as anything the police here have ever seen," Mr. Walczak said.

So what are the police planning?

Unprecedented security preparations likely will transform the streets of Downtown Pittsburgh during the gathering. American cities have known disruption before, although usually not over economic disputes. A notable exception was 1999, when thousands of anti-globalization protesters clashed with police in Seattle during a meeting of the World Trade Organization.

That episode startled the nation, led to the resignation of Seattle's police chief and put the economy on the map as a stamping ground for potentially violent demonstration.

Mayor Luke Ravenstahl said he knows the G-20 poses some security problems likely to be felt by Pittsburghers.

"Of course, it will be an inconvenience in some ways," he said. "But it's an opportunity for the residents of this city to experience something they've never seen before."

It's also possibly something that city and regional law enforcement agencies have never seen before -- certainly not on this scale.

Because of the event's collection of VIPs, the federal government may designate it as a "National Special Security Event." That means the Secret Service will take the lead for planning security.

"Our job is to be as unobtrusive as possible for people who live and work in Pittsburgh," said Special Agent Jim Mackin, an agency spokesman in Washington, DC.

Much of the organizing will take place in its Downtown Pittsburgh office, headed by Special Agent in Charge Jim Gehr.

"We quite frankly rely very heavily on our state and local partners," said Agent Gehr, who has 26 years of experience with the Secret Service. "We absolutely know each other well. My feeling is they are top notch."

Conversations between local and federal officials already are under way, but, not surprisingly, those officials are unwilling to discuss specifics.

"The security for this event is similar to others. The basic principles are the same. It's just a lot more high profile and international," city Public Safety Director Michael Huss said. "We will have a plan to deal with it."

The city will have to make new purchases, including sizeable quantities of tear gas. Federal money will cover some of the costs of equipment and overtime pay for many officers.

City police officials regularly coordinate with dozens of other law enforcement agencies throughout the Region 13 Task Force, which includes counties in Western Pennsylvania and has its headquarters at the Allegheny County 911 center in Point Breeze.

"Obviously our emergency operations center, county police and sheriff will all work with the federal government," said county spokesman Kevin Evanto.

The task force has already worked on other major events, such as the 2006 baseball All-Star game. And local police have tested their crowd control skills in recent months, including during the post-Super Bowl revelries on the South Side and in Oakland.

Authorities are looking to what happened last month at the G-20 meeting in London, when thousands of protesters took to the streets and clashed with some of the estimated 10,000 police assigned to the meeting.

British police received heavy criticism for their handling of the protests, including the practice of "kettling," or keeping large groups of people confined in one area for hours at a time.

The London protests, while centered around a meeting over economics, included a panoply of diverse groups. Some protested climate change, others human rights, still others poverty.

"They didn't have a unified message in London the way they have had in other places," said Colin Bradford, a senior fellow at the Brookings Institution and a proponent of the G-20.

Sometimes lost amid the video of masked protesters dodging police lines and navigating clouds of tear gas is the brew of political, economic and environmental objections that underlie those protests.

The common link among those groups is a shared sense of being locked out of the process.

"It's hopelessly undemocratic," said Noam Chomsky, a renowned linguist, activist and anarchist author who has been a central figure in contemporary opposition to global corporations.

"The people that appear there have no accountability to the public except very indirect. It's unaccountable and they are not acting in the benefit of the public, nor would you expect them to be. They are acting on behalf of their constituency, which happens to be the rich sectors."

Mr. Chomsky said he turned down an invitation to speak at an anti-G-20 protest in Pittsburgh. He couldn't remember which group extended it, but that it was already offered suggests some opposition has begun coalescing.

Clearly, not everyone shares this sense of exclusion.

Laura E. Ellsworth, a partner at the Downtown law firm of Jones Day, which has offices in 11 of the G-20's countries, expects protests, but primarily because the attention focused on the meeting provides a stage.

"I think they make a fuss because they can, because it's a gathering that quite properly draws international attention," Ms. Ellsworth said.

She is skeptical of complaints about the G-20 being undemocratic.

"Government policy almost by definition is set by governments. In this country, that government is supposed to be representative of the voices of the people," she said.