The orange alert instituted two weeks ago for U.S. mass transit systems is likely to be prolonged after the latest terrorist attacks in London.

Homeland Security Secretary Michael Chertoff (search) and his staff discussed the mass transit alert levels several times Thursday, before and after the attacks, said two counterterror officials who spoke on condition of anonymity because of the sensitive nature of the information.

At the first meeting, senior national security officials were described as leaning toward dropping the transit system from orange, or high risk, back to yellow, signifying an elevated risk, although no final decision was made. By early afternoon, however, White House spokesman Scott McClellan said, "There is no plan at this time to change the alert level for our mass transit systems."

No one was injured in the attempted bomb attacks Thursday on three subway trains and a double-decker bus in London. Still, British Prime Minister Tony Blair (search) said: "We can't minimize incidents such as this. They're done to scare people, to frighten them and make them worried."

Homeland Security Assistant Secretary Brian Besanceney said, "Certainly, as we look at the threat level, we will factor in the information and intelligence we're receiving from the U.K., as well as information we're getting from other sources, as we consider what the right protective measures are." He added that the department has reviewed the alert levels daily since they were raised to code orange on July 7.

The rest of the country remains at yellow alert, and counterterror officials said they have not seen any specific intelligence that would indicate an attack on the United States.

Maintaining security at orange alert levels — with increased police patrols, heightened inspection and surveillance and the use of bomb-sniffing dogs — costs mass transit systems an estimated $900,000 a day, said Greg Hull, security chief for the American Public Transportation Association (search).

Leaders on both sides of the Atlantic urged citizens to stand firm against attacks that they described as designed to scare people.

President Bush, speaking to members of the Organization of American States, said terrorists "don't understand that when it comes to the defense of universal freedoms, this country won't be frightened." He did not specifically mention the explosions in London but devoted part of his speech to the war against terrorism.

"They have territorial designs," the president said. "They have the desire to use their terrorist techniques to frighten us. They understand when they kill in cold blood it ends up on our TV screens and they're trying to shake our will."

"We will defend ourselves by staying on the offense against these killers," Bush said. "We will find them overseas so we don't have to face them here at home."

Some federal facilities stepped up their security measures, including the Pentagon, which is adjacent to a Washington-area subway stop, and the U.S. Embassy in London, which was temporarily closed to the public except for emergencies.

In Washington, subways and buses were running normally and authorities remained on heightened alert, Metro spokeswoman Lisa Farbstein said.

The London explosions triggered immediate reactions on Capitol Hill, where the House chaplain opened the day's proceedings with a prayer for any victims and one congressman called for additional funding for mass transit security.

"Instead of acting as a wake-up call, Congress seems to be hitting the snooze button," said Rep. Bob Menendez, D-N.J. "How many warnings do we need before we take action?"

But officials generally urged Americans to continue with their daily routines. Chertoff, for example, was keeping plans to leave Washington in the afternoon to meet with local officials Friday in St. Paul, Minn., a spokesman said.