LOS ANGELES — Gunfire echoed through downtown and law enforcement helicopters swooped low among office towers Thursday, but it was all a drill as the Police Department's counterterrorism unit demonstrated a response to a weapon-of-mass-destruction threat.

The late-morning demonstration began with an explosion of flash grenades, officers firing blank ammunition at pretend suspects, and police rappelling out of a county Sheriff's Department helicopter and onto a hotel bridge on Figueroa Street.

A red pickup truck carrying a barrel drove onto the scene shortly after, presenting officers with the challenge of neutralizing an improvised explosive device in the center of a highly populated area.

The drill was planned in conjunction with the National Homeland Security Association's conference at the Bonaventure Hotel and was conducted by the Police Department's Counter Terrorism and Special Operations Bureau.

Police Chief Charlie Beck said about 1,800 people from 60 major cities attended the conference and most watched the demonstration. Representatives from the United Kingdom, New Zealand, Australia and Canada also attended.

LAPD spokesman Cmdr. Andrew Smith said the drill strengthens links and coordination within the department and other authorities.

"We know that terrorists are capable of acting everywhere," Smith said. "We need to maintain our vigilance. We need to maintain our edge. That's the reason we do this training."

About 50 officers from various units, mainly SWAT, air support and hazardous materials, participated in the drill, following months of preparing and notifying area businesses of the exercise.

Beck added that the downtown demonstration was the second part of a drill that began Wednesday night and included Long Beach police, the Sheriff's Department and FBI in a water-borne scenario in the San Pedro harbor area.

Beck said the annual exercise plays a key role in preparing for a domestic terrorist attack in the wake of tragedies like the Boston marathon bombing and the July 2012 theater shooting in Aurora, Colo.

"This is something we constantly worry about. The threat is always there," he said in a statement. "What happens in other regions of the world is important, but we have enough of a homegrown issue that all this needs to be practiced continuously."

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