PITTSBURGH — Worried about protests that typically surround the meetings of officials from the world’s 20 biggest industrial and emerging economies, Pittsburgh has asked 4,000 officers from across the country to supplement its 900-member police force for the Group of 20 meeting here next month.

But the advocacy groups that have been planning to march, form tent cities and demonstrate worry that the heightened security will itself cause a problem.

Officials from Pittsburgh and Allegheny County, along with a White House advance team and other federal officials, visiting the site where the economic meeting will be held in September.

“That’s an overreaction, in my view,” said Molly Rush, co-founder of the Thomas Merton Center, a social advocacy organization here that is planning a protest march on Sept. 25, the second day of the two-day G-20 meeting. “What could cause a problem isn’t the protesters, but an overreaction of security forces.”

The city has tried to accommodate the protest groups, deciding on Friday to grant six permits for demonstrations and marches after previously saying most of them would be denied.

The city has also promised at least two protest spots “within sight and sound” of the David L. Lawrence Convention Center downtown, where the G-20 meetings will be held, avoiding the “free-speech cages” that were set up in some cities far away from, for example, the Democratic and Republican national convention sites.

But in announcing the permit approvals, Mayor Luke Ravenstahl also proposed ordinances to allow the police to clamp down on some protest techniques. Masks would be banned, as well as PVC pipe, which some protesters use to link hands and make separating them more difficult, if they are used “during the commission of unlawful activity.”

Pittsburgh estimates that it will cost $19 million to provide security for the meeting. The federal government has agreed to provide $10 million. The state will kick in $6.3 million, including $1.8 million in the in-kind contribution of 600 state troopers as part of the 4,000 additional officers. The remaining $2.7 million will be paid by the city and Allegheny County.

Even with the promised protest spots, advocacy groups remain concerned about the size of the security perimeter the federal government is going to set up around the convention center.

“To be consistent with this message of allowing free speech, the perimeter can’t be 10 blocks around the convention center, like we’ve been hearing it might be,” said Witold Walczak, legal director for the American Civil Liberties Union of Pennsylvania, which has been advising many of the protest groups. “The way Pittsburgh’s awkward street grid is set up, that would be virtually the whole downtown.”

The city acknowledges that the labor, environmental, social justice and religion-based groups that have applied for permits are generally seen as peaceful.

But Michael Huss, the city’s public safety director, said the city and the Secret Service, which oversees security plans because the conference has been declared a National Special Security Event, were “planning for the worst-case scenario.”

For meetings like the G-20, that means something along the lines of what happened during the World Trade Organization protests in Seattle in 1999, when about 50,000 protestors managed to shut down the meeting for a time and grab headlines around the world, or the G-20 meeting in April in London, when 35,000 people protested.

Mr. Huss said Pittsburgh officials had visited or spoken with security officials in Boston, Denver, London, New York, St. Paul and Washington, which have all hosted large gatherings with protesters.

In addition, Mr. Huss said he recently read the book “Breaking Rank” by Norm Stamper, which includes Mr. Stamper’s view of the mistakes that were made during the W.T.O. conference when he was the police chief in Seattle.

In an interview, Mr. Stamper said Pittsburgh had already corrected for one big mistake he had made and can learn from another.

“I think Pittsburgh is very smart to ask for those 4,000 officers,” he said.

He still blames himself for allowing the police to use tear gas to break up a street demonstration on the first day of the W.T.O. meeting, which he believes set the tone of conflict for the rest of the week.

“It was the cop in me who O.K.’d the use of chemical agents, but the police chief in me should have vetoed the decision,” he said.

Pittsburgh has a small, active organization of anarchists who have established a temporary Web site, resistG20.org.

“Our goals are less about trying to prevent 20 people from meeting than to challenge the presence of the meeting,” said Joanna Tamborino, a spokeswoman for the Pittsburgh Organizing Group, an anarchist organization with about 300 members that will oppose the G-20 conference.

Some blogging anarchists say that with the recent financial meltdown, the G-20 event in Pittsburgh, coming almost 10 years after Seattle, could provide the perfect timing for another major anarchist action. But few believe anything along those lines will occur here.

“With our success there, we gave up the element of surprise,” said John Sellers, president of the Ruckus Society, a group that helped train many of the protesters for the W.T.O. protests in Seattle. “I don’t think in our lifetime we’ll have a large city police force be taken by surprise like that again.”

www.nytimes.com/2009/08/23/us/23pittsburgh.html