PITTSBURGH - Security officials say they've thought of everything possible to keep the land, water and air around the Group of 20 economic summit safe.

The discovery of an alleged bomb plot in Denver and New York set off a series of counterterrorism bulletins - warnings that come as security officials finalized plans for the gathering of world leaders in Pittsburgh on Thursday and Friday.

Asked whether the terror alert was affecting preparations in Pittsburgh, Secret Service spokesman Darrin Blackford said only that the agency and its partners were "preparing for all contingencies." But the agency's head in Pittsburgh said the focus should be on the G20, not the security preparations.

"Though security is a part of this event, it's something we want to put in the background," said Special Agent James Gehr.

On Tuesday, the officials showed off a communications centre for more than 50 federal, state and local agencies that will be the nexus of efforts to keep diplomats and the public safe. The agencies represented include the National Geospatial Intelligence Agency - which can gather satellite pictures of the event.

The communications centre will be staffed from Wednesday evening until Friday evening. The media has agreed to keep its location a secret.

Gehr acknowledged that despite efforts to keep security in the background, many downtown businesses will close because employees or customers are hampered by strict vehicle restrictions near the David L. Lawrence Convention Center, or the fear that expected protests could turn violent.

Gehr said officials had little choice but to ban almost all vehicles downtown to prevent gridlock that could have blocked numerous bridges and tunnels that funnel traffic into downtown, which rests in a triangle bounded by two rivers.

Mayor Luke Ravenstahl encouraged people to come downtown.

"You can get into downtown Pittsburgh. It will be a normal business day for many businesses," the mayor said. "Everybody should feel safe in downtown Pittsburgh Thursday and Friday."

Although delivery vehicles will be allowed in most downtown areas, the U.S. Postal Service on Tuesday nonetheless said it will close two main downtown branches Wednesday through Friday due to traffic restrictions. Mail delivery to closed-off areas would be attempted, the agency said.

The Coast Guard is also closing the city's three rivers from 6 a.m. Thursday to 10 p.m. Friday and patrolling with 11 boats, Petty Officer Second Class Terrell Sisk said. The closed area includes the confluence of the Allegheny, Monongahela and Ohio rivers and encompasses a five-mile Y-shaped area along them.

State police will have 1,200 troopers in the area, including 20 mounted police, a special bicycle unit and helicopters and airplanes. The Pennsylvania National Guard is heading a task force of 2,500 troops and other Department of Defence personnel for crowd control and other duties.

Despite the tight security, the summit has created "legions and dozens of fantastic, positive stories about Pittsburgh," said Craig Davis, vice-president of sales and marketing for VisitPittsburgh.com.

Some businesses have boarded up their facades, but Allegheny County Chief Executive Dan Onorato said the vast majority have not.

On Tuesday afternoon, workers were erecting chain-link fences around the headquarters of PPG Industries Inc., a series of buildings sheathed in the company's glass.

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Associated Press writers Vicki Smith and Ramit Plushnick-Masti in Pittsburgh contributed to this report.