NEW YORK —  The city is strengthening its already-tight security as a precaution after a bomb was discovered in a busy area of London, Mayor Michael Bloomberg said Friday.

"We're going to ramp up a little bit, but nothing dramatic," Bloomberg said on his weekly radio show. "We'll take a little bit of extra precaution. Some of you will notice, some of you won't — but we have to be cognizant."

The U.S. government urged Americans to be vigilant about suspicious activity after British police defused the bomb in central London, but officials said they saw no potential terrorist threat in the United States ahead of next week's Fourth of July holiday.

In New York, police were placing more patrols in Times Square and the mass transit system, according to department spokesman Paul Browne. Officers were working beyond the end of their shifts to provide extra coverage during the evening rush hour.

Bloomberg noted that the New York Police Department has an officer assigned full time in London. "He is there and he'll keep us posted," the mayor said. "We will have our police officer give us a full report, because you want to know what happened."