(Adds more from police commissioner, security video)
By Emily Chasan and Walker Simon
NEW YORK, March 6 (Reuters) - A small explosion damaged a U.S. military recruiting station but caused no injuries in New York's Times Square before dawn on Thursday, triggering a Pentagon alert for other stations across the country.
"We're treating it as if it were an incident of vandalism," Army spokesman Paul Boyce said at the Pentagon.
Times Square -- the normally bustling "Crossroads of the World" with shops, restaurants, hotels, theaters and office towers -- was largely empty when the crude bomb went off at around 3:45 a.m. (0845 GMT).
Low-grade explosives packed in an ammunition box cracked the recruiting station's thick glass door and twisted its metal framing, police said. The blast also shattered a window encasing the classic poster of Uncle Sam saying "I Want You."
In Washington, the Homeland Security Department said there was no sign of an immediate threat to the United States from the incident and White House spokeswoman Dana Perino said there was no initial sign of any link to terrorism.
The Army sent a notice to its 1,650 recruiting stations nationwide to remind recruiters to be careful, Boyce said.
New Yorkers have been on alert since al Qaeda militants used hijacked planes to destroy the World Trade Center towers in Manhattan on Sept. 11, 2001, killing more than 2,700 people. The Twin Towers were also targeted in 1993 by a truck bomb that killed six people.
Police Commissioner Ray Kelly showed reporters a security camera video of the blast and said police were looking into similarities to small explosions at the British and Mexican consulates in 2005 and 2007, also in the early morning hours.
The video was dark and grainy, but Kelly said it showed a person on a bicycle approaching a traffic island, dismounting and walking to the door of the recruiting station. The person leaves shortly before the blast.
A witness described the suspect as "fairly large" but could not say for sure whether it was a man or a woman, Kelly said.
The one-story recruiting center in a traffic island in the middle of Times Square invites people to sign up for the U.S. armed forces and periodically attracts anti-war protesters.
The Granny Peace Brigade, an anti-war group that has frequently held protests at the site, denounced the bombing and disassociated itself from "mindless acts of terror."
The blast occurred before the fifth anniversary on March 19 of the start of the U.S.-led invasion of Iraq.
Mayor Michael Bloomberg said the attack was an insult to all military personnel serving around the world.
A witness told police of a hooded person with a backpack riding a bicycle "in a suspicious manner" in front of the station shortly before the blast, Kelly said.
The bomb was larger than those used in the grenade attacks on the British and Mexican consulates, Kelly said. He said it could have caused injury or even death.
In May 2005, the British consulate in New York was attacked by two small pre-dawn blasts from two "novelty" grenades in the shape of a pineapple and a lemon. Last October, the same types of grenades were used in blasts at the Mexican consulate.
In both incidents, a man was seen fleeing on a bicycle.
Kelly said he could not definitively say Thursday's blast was linked to the two consulate attacks but it was "certainly a possibility" given the similarity in the time of day, sighting of a person on a bicycle and size of the blasts.
He said forensic teams would investigate whether they were in fact linked.
Nino Reyes, 26, told Reuters he had just opened his coffee and snack stand when he heard an explosion and saw a plume of smoke shoot up. (Additional reporting by Yinka Adegoke and Claudia Parsons in New York and Tabassum Zakaria and Randall Mikkelsen in Washington; Writing by Daniel Trotta; editing by Patricia Zengerle)