Security was tightened at malls Thursday after an anonymous caller warned federal authorities that terrorists could strike a Los Angeles shopping center near the Federal Building.

Dozens of police and security guards patrolled malls close to the facility, including the indoor Westside Pavilion and The Grove, a maze of new shops in the Fairfax District.

Mayor James Hahn (search) urged people to continue their usual activities while remaining vigilant for unusual activity like trucks parked too long in one place or people wearing bulky clothing in hot weather.

Authorities have previously acknowledged a number of sites in Southern California could be targets of terrorism, including Los Angeles International Airport (search), ports, movie studios and bridges.

Police said they have fielded more than 1,000 tips about possible threats in the past year — but decided to make this one public because it offered a specific day and general target.

A caller told federal officials several days ago an unnamed mall near the Federal Building could be struck Thursday, but didn't describe the nature of the attack, police said.

"The information is uncorroborated and the credibility of the source is unknown," the Los Angeles Police Department said in a statement.

Police Chief William Bratton said the call was analyzed for several days along with other information that he declined to specify. Malls were notified and police were placed on citywide alert before the warning was made public Wednesday night.

The alert was "strictly precautionary," Bratton said during a news conference.

"People are going to have to get used to these," he said. "It's the reality of the world that we're living in."

Police increased patrols at shopping malls and asked mall operators to beef up security while a joint terrorism task force investigates.

Tom Miles, The Grove's general manager, said shopping traffic appeared normal. "Life goes on," he said, adding that the mall never considered closing.

LAPD officers also patrolled the Westside Pavilion, about 21/2 miles away from the Federal Building. Early morning crowds seemed sparse, although mall marketing manager Erica Boatman-Dixon said it appeared to be a typical midweek turnout.

Corinne Levin, 37, took her son and nephew, both 5, to the mall's play area, where just a handful of children gathered. "I'm concerned, but I'm not rushing out, either," she said. "If you're going to make a point, why bomb a deserted mall?"

Movie crew members putting up sets for a new Tim Allen movie called "Skipping Christmas" shrugged off the warning.

"We can't let it bother us. We can't live in fear," said Alex Barnoya, 42.