Police on Thursday began random searches of bags and packages carried by people entering city subways, a new security measure announced after another round of bomb attacks in London.

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

The inspections began Thursday on a small-scale basis in at least one location in Manhattan, where a cluster of officers was seen stopping five men over a 15-minute period as they entered a subway at evening rush hour. In each instance, the officers peered briefly into the men's bags, then waved them through.

Full-scale inspections are scheduled to be in place by rush hour Friday.

Passengers carrying bags will be selected at random before they pass through turnstiles, and those who refuse to be searched won't be allowed to ride, Police Commissioner Raymond Kelly (search) said.

The announcement drew complaints from civil liberties (search) advocates in a city where an estimated 4.5 million passengers ride the subway on an average weekday. The system has more than 468 subway stations — most with multiple entrances — and the flood of commuters hurrying in and out of stations during rush hour can be overwhelming.

Kelly stressed that officers posted at subway entrances would not engage in racial profiling, and that passengers are free to "turn around and leave." He also downplayed the possibility of bottlenecks at subway entrances.

Officials declined to specify how frequently the checks would occur. Authorities said bus and commuter train passengers will also be checked.

"If it serves a purpose, I'm OK with it," said one of the men inspected Thursday, James Washington, 45.

William K. Williams, a 56-year-old Manhattan resident who rides the train every day, said such security measures — while inconvenient — are a way of life now.

"It doesn't bother me — I mean, the whole state of things bothers me — but it's just part and parcel of the world we live in," said Williams, who was carrying a briefcase outside the Brooklyn Bridge station of the subway.

The New York Civil Liberties Union said the searches violate basic rights and will inconvenience New Yorkers, but the group stopped short of threatening a legal challenge.

"The NYPD can and should investigate any suspicious activity, but the Fourth Amendment prohibits police from conducting searches where there is no suspicion of criminal activity," executive director Donna Lieberman said.

Andrew Morris, a 57-year-old New Yorker who had a large bag slung over his shoulder Thursday, said he would consent to a search if asked, but added that the extra security measures are essentially useless.

"I think these terrorists go where it's easiest to go, so if you make it hard on the subway, they'll go where we're weak," Morris said.

Williams predicted the new searches would frustrate New Yorkers, not exactly known for their patience.

"Sometimes you need to get to an appointment, you're running late and a cop stops you to delay you even further? That's going to create a mess," he said.

In Boston, additional police officers were added at downtown subway stations after the latest round of London explosions, and Gov. Mitt Romney boarded a subway to reassure residents that the city's transit system is safe.