NEW YORK - Police will begin conducting random searches of packages and backpacks carried by people entering city subways, Mayor Michael Bloomberg announced Thursday after a new series of bomb attacks in London.

Authorities said the system is still being developed, but the plan is for passengers carrying bags to be selected at random before they have passed through turnstiles.

Police Commissioner Raymond Kelly promised that officers would not engage in racial profiling, and that passengers will be free to "turn around and leave" rather than consent to a search.

Officials would not immediately say how frequently the checks would occur. The checks are scheduled to begin at some stations by Thursday evening and will be occurring throughout the system by rush hour on Friday.

"We just live in a world where, sadly, these kinds of security measures are necessary," Bloomberg said. "Are they intrusive? Yes, a little bit. But we are trying to find that right balance."

Searching the bags of more than a token number of straphangers may be impossible.

New York's subways carry about 4.5 million passengers on the average weekday, according to the Metropolitan Transportation Authority.

There are 468 subway stations in the system, most of which have multiple entrances, and during rush hours, the flood of commuters hurrying in and out of key stations can be overwhelming.

Asked whether the searches might create bottlenecks at subway entrances, Kelly suggested the searches would be of a small enough sampling of passengers that only individuals, rather than whole crowds, would be delayed.

"We are going to do it in a reasonable commonsense way," he said.

Similar types of random searches of subway passengers have prompted complaints from civil liberties groups in other cities, and in some cases have been challenged in court.

Christopher Dunn, associate legal director of the New York Civil Liberties Union, said the searches in New York could be problematic, if not conducted properly.

"The department can and should be actively and aggressively investigating anyone they suspect of bringing explosives into the subway, but police searches of people without any individualized suspicion is contrary to our most basic constitutional values," he said.

Authorities said there is also a possibility that checks will be conducted on some bus and train passengers.