When revelers pack Times Square for the annual New Year's Eve celebration tonight, police will observe a tradition of their own: giving them lots of company.

Each year, the New York Police Department assigns thousands of extra patrols to festivities -- in ways seen and unseen -- to control the crowd and watch for any signs of trouble. Hundreds of thousands of people from all over the world are expected to pack into the bow-tie stretch of streets in midtown Manhattan to see the crystal ball drop and ring in 2013.

"We think it's the safest place in the world on New Year's Eve," NYPD Commissioner Ray Kelly said.

Security in Times Square has become an obsession for the NYPD in the post-9/11 world, especially since an attempted car bombing there in summer 2010. More recently, details emerged in another case in Florida in which one suspect considered Times Square as a potential target.

"Times Square is an iconic location that draws a significant number of people every day," Kelly said, adding that there are no specific terror threats. But the nation's largest police department has turned securing the event into a science.

The department has worked closely with hotel managers, urging them to guard against anyone who might seek to check into a guest room and use it to launch an attack.

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Last year, police noticed that revelers started flocking to Times Square earlier in the day to hear rehearsals of performers scheduled for various telecasts. So this year, police will adjust by posting more officers on the streets before nightfall, he said.

Along with the uniformed officers, police will use barriers to prevent overcrowding, inspect vehicles, enforce a ban on alcohol and check handbags. Visitors will see bomb-sniffing dogs and heavily armed counterterrorism teams. Rooftop patrols and NYPD helicopters also will be deployed.

Other plainclothes officers are assigned to blend into the crowd. Many officers will be wearing palm-size radiation detectors designed to give off a signal if they detect evidence of a dirty bomb. The bomb squad and another unit specializing in chemical and biological threats will sweep the area. They also will patrol the sprawling Times Square subway station.

Another practice: Sealing manhole covers and removing mailboxes to keep anyone from using them to conceal a bomb.