Washington’s Metro transit agency tightened security Thursday in response to the latest London bombings, increasing patrols and planning a sweep of the entire subway system Thursday night to search for explosives and weapons.

But Washington did not go as far as New York City, which today is scheduled to become the first city in the nation to begin randomly checking bags at its subways, commuter railways and buses.

“We just live in a world where, sadly, these kinds of security measures are necessary,” New York Mayor Michael Bloomberg 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 today.

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 said.

The announcement drew complaints from civil liberties 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.

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.