May 5 (Bloomberg) -- Explosives packed in two replica hand grenades detonated early this morning outside the entrance to a glass-and-steel office tower in New York City housing the U.K. consulate, authorities said.

The Associated Press, citing law enforcement sources, said investigators were questioning a United Nations employee from the Netherlands found loitering near the building shortly after the explosion.

No one was injured and there were no claims of responsibility for the 3:35 a.m. blast, Mayor Michael Bloomberg told reporters at the site, near the Citigroup Center in midtown Manhattan.

``We at this point have absolutely no knowledge what the motive was,'' Bloomberg said. ``No one should jump to conclusions.''

The 21-story building at 845 Third Avenue, spanning the block between 51st and 52nd Streets, is home to a variety of foreign and domestic businesses. The street-level retail space is shared by a branch of North Fork Bank, based in Melville, New York, and a sporting goods chain, The Sports Authority, based in Englewood, Colorado.

The explosion came on a day when Britons were voting in parliamentary elections, and on the anniversary of the 1981 death of Irish Republican Army member Bobby Sands following a hunger strike that sparked riots in Northern Ireland.

Conference Board

The British consulate's presence isn't advertised on the exterior of the building, which also houses the and the U.S. headquarters of Asahi Shimbun, Japan's second-largest newspaper, and the Conference Board, a research group that produces a monthly U.S. consumer confidence index and an index of leading economic indicators, among other U.S. economic barometers.

The two ``novelty-type'' grenades, placed in a concrete flower planter outside the building's entrance, had evidently been stuffed with black powder and detonated using a crude fuse, Police Commissioner Raymond Kelly told reporters. They blew an hourglass-shaped hole in a glass panel between two revolving doors.

``If someone was in the vicinity, they could have been seriously injured,'' Kelly said.

Police closed Third Avenue between 50th and 53rd Streets, and subway trains skipped the 51st Street stop on the Lexington Avenue line for several hours while the blast was being investigated. Other consulates in the city were checked as a precaution and no other devices were found, Bloomberg said.

The UN tightened security at all entry points and called in about a half-dozen bomb-sniffing dogs to search for possible explosives, spokesman Stephane Dujarric said.

Outside the building this morning, Sayan Murkherjee, 35, of Plainsboro, New Jersey, said he was ``pretty shocked'' when he heard of the bombing, but decided to take a chance and go to the consulate for a visa anyway.

`London Was Open'

``I thought that London was open for business during the blitz, so I thought that a couple of firecrackers should not bother them,'' Murkherjee said.

The U.S. Federal Bureau of Investigation is investigating the incident along with the New York police bomb squad, the police arson and explosion squad and a joint local-federal terrorism task force, a police statement said.

Police officers from the 17th Precinct stationhouse, around the corner from the building, and firefighters from a nearby fire station heard the explosions and ran to the scene. They didn't see anyone running away, Bloomberg said.

The mayor is founder and majority owner of Bloomberg News parent Bloomberg LP.

To contact the reporters on this story: Todd Zeranski in New York at tzeranski@bloomberg.net; David M. Levitt in New York at dlevitt@bloomberg.net;

Last Updated: May 5, 2005 13:06 EDT