By David Johnston and Eric Schmitt

updated 8:57 a.m. ET Aug. 5, 2008

WASHINGTON - Federal and local authorities are girding for huge protests, mammoth traffic tie-ups and civil disturbances at the Democratic National Convention in Denver this month, fearing that the convention will become a magnet for militant protest groups.

Officials say that what makes Denver different than past conventions is the historic nature of Senator Barack Obama’s nomination, a megawattage event whose global spotlight could draw tens of thousands of demonstrators, including self-described anarchists who the police fear will infiltrate peaceful protest groups to disrupt the weeklong event.

The Secret Service is wary of discussing threats against the people they protect, but with Mr. Obama poised to become the first black presidential nominee, there are special worries. While law enforcement officials say there are no specific, credible threats against Mr. Obama, they expressed concern about low-level chatter on Web sites frequented by white separatists who spew hate about Mr. Obama’s race and what they perceive as his liberal agenda.

One recent scheduling change caused a major shift in security plans. When Mr. Obama announced last month that he would accept his party’s nomination not at the Pepsi Center in downtown Denver, where the convention is being held, but at Invesco Field, home of the Denver Broncos, the Secret Service scrambled to work out plans with local authorities to secure the open-air stadium, which seats more than 75,000 people. Invesco is also adjacent to Interstate 25, a major corridor through the Northern Rockies that will most likely be closed for at least part of Mr. Obama’s acceptance speech.

“The magnitude of the event has expanded,” said John W. Hickenlooper, the mayor of Denver and a Democrat. “It’s bigger and more profound than we expected.”

Cannot take any chances
Officials acknowledge that their projections for the number of protesters are based more on a worst-case chain of events than specific information about who will show up, but they say they cannot take any chances.

As a result, the Secret Service, the Pentagon, the Federal Bureau of Investigation and scores of police departments are moving thousands of agents, analysts, officers and employees to Denver for the Aug. 25-28 convention. They will operate through a complex hierarchy of command centers, steering committees and protocols to respond to disruptions.

National political conventions are a chance for federal agencies to test their latest and most sophisticated technology, and this year is no different. There was a brief flare-up recently between the F.B.I. and the Secret Service, when each wanted to patrol the skies over the convention with their surveillance aircraft, packed with infrared cameras and other electronics. The issue was resolved in favor of the Secret Service, according to people briefed on the matter.

Enlisting extra officers
Both Denver and St. Paul, where the Republican National Convention will be held Sept. 1-4, are enlisting thousands of additional officers to help with security. Even so, their numbers will be only about a third of the 10,000 police officers that New York City fielded for the 2004 Republican convention, just three years after the Sept. 11 attacks.

The Denver Police Department will nearly double in size, according to federal officials involved in the planning. The city is bringing in nearly 1,500 police officers from communities throughout Colorado and beyond, even inviting an eight-person mounted unit from Cheyenne, Wyo. State lawmakers changed Colorado law to allow the out-of-state police officers to serve as peace officers in Denver.

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The expressions of concern about security at the convention could have more immediate political and legal implications, too. A federal judge, Marcia S. Krieger of United States District Court in Denver, is expected to issue a decision this week in a lawsuit filed by the American Civil Liberties Union seeking to ease security provisions at the convention. The A.C.L.U. has suggested that the Secret Service and the Denver police have exaggerated risks as part of a crackdown on dissent.

The case centers on whether the security zone around the Pepsi Center is so large, and the designated parade route through the city for marches and rallies so far away, as to unnecessarily stifle free speech. New worries about protests and anarchy could bolster the government’s case that the plans are justified.

Last month, under pressure from the A.C.L.U. lawsuit, the city released a list of expenses related to the convention showing that the police were preparing for large demonstrations and mass arrests and that the department had spent $2.1 million on protection equipment for its officers, $1.4 million for barricades and $850,000 for supplies related to the arrest and processing of suspects.

In disclosing the cost breakdown, city officials denied rumors that had circulated for weeks that they had contemplated buying exotic nonlethal weapons that fired an immobilizing goo, or that used radiation or sonic waves to incapacitate people or vehicles.

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