"They've got to be right one time and we've got to be right 100 percent of the time to protect the American people," Mr. Bush said.
The president hailed the disruption last week of the plot to blow up U.S.-bound airliners with liquid explosives. He said because of the counterterrorism efforts of the United States and its allies, "America is safer than it has been, yet it is not yet safe."
The counterrorism center is located at an undisclosed site in suburban northern Virginia, known as Liberty Crossing. It merges hundreds of government experts and more than two dozen computer networks from various federal agencies focusing on potential threats.
Its high-tech, 24-hour operations center is among the most sophisticated in the U.S. government.
U.S. safety looms as a major issue in the midterm elections Nov. 7. Both Republicans and Democrats are maneuvering for the political advantage in an election where control of Congress is at stake.
Democrats have been accusing the Bush administration of trying to ignite fear among Americans and gain political points by claiming they alone can keep them safe. Republicans argue that Democrats are weak on national security.
Mr. Bush credited American counterterrorism workers with helping to bring about last week's arrests of more than two dozen people in England and Pakistan in what officials say was a plot to blow up as many as 10 passenger planes between Britain and the United States.
He said their work takes "an incredible amount of time, energy and effort.
"That plot and this building and the work going on here is really indicative of the challenge we face — not only this week, but this year and the years to come."
Mr. Bush said the United States was engaged "in a war against a extremist group of folks, bound together by an ideology, willing to use terrorism to achieve their objectives," He spoke as police in London announced another arrest in the alleged London airliner bomb plot.
"I will assure the American people that we are doing everything in our power to protect you," he said.
Mr. Bush was spending the better part of his day at the center where employees watched large screen monitors and surveyed electronic maps of various parts of the world. He received a briefing from his National Security Council and Homeland Security Council and had separate sessions with each team, attending lunch with all the officials in between.
On his handling of foreign policy and terrorism, Mr. Bush was at 40 percent approval in an early August AP-Ipsos poll. That is near his all-time low of 39 percent on those issues in AP-Ipsos polling.