"We will no doubt face new challenges. But we have our marching orders. My fellow Americans, let's roll," he told a cheering audience.

In a prime time national address — carried live by only one broadcast network — Mr. Bush said the nation has risen to the challenges of war on terrorism, fighting to preserve the best of America.

"Many ask, 'What can I do to help in our fight?' The simple answer is all of us can become a September 11 volunteer by making a commitment to service in our own communities," Mr. Bush said.

"One way to defeat terrorism is to show the world the true values of America through the gathering momentum of millions of acts of responsibility and decency and service," he said, prompting a standing ovation from the audience of about 5,000 people gathered at the Georgia World Congress Center.

Mr. Bush won the loudest applause at the end, when he praised the actions of passengers who fought hijackers aboard United Airlines Flight 93 before it crashed into a Pennsylvania field. Mr. Bush recalled the words of Todd Beamer, a 32-year-old Sunday school teacher, who was overhead on a cellular phone to say, "Let's roll" as passengers charged the terrorists.

Mr. Beamer talked to a cellular-phone company supervisor for about 13 minutes before his plane crashed into a field in western Pennsylvania. He told Lisa Jefferson that the passengers had decided they would not be pawns in the suicidal destruction of a national monument. Before going off to foil the hijackers, Mr. Beamer made Miss Jefferson promise to call his wife and two children.

In his 32-minute speech, Mr. Bush proposed mobilizing 20,000 Senior Corps and AmeriCorps participants to support homeland security. Under the plan, 10,000 new volunteers would support police departments and other local agencies, freeing up local officials needed in the case of a terrorist attack.

The plan also calls for 5,000 volunteers to support public health agencies and another 5,000 to respond to areas hit by terrorism, working with federal agencies to help those most affected.

"We will ask state and local officials to create a new, modern civil-defense service — similar to local volunteer fire departments — to respond to local emergencies when the manpower of governments get thin," Mr. Bush said. "We will find ways to train and mobilize more volunteers to help when rescue and health emergencies arise."

The president said the act of volunteering will send a message to those in the world who view Americans as "shallow, materialistic consumers who care only about getting rich or getting ahead."

"This isn't the America I know," he said.

Mr. Bush also announced the creation of a presidential task force on "citizen preparedness" in the war on terrorism and directed it to make recommendations in 40 days to help prepare Americans for terrorism in their homes, neighborhoods, schools, workplaces, places of worship and public places.

Homeland Security Director Tom Ridge will be co-chairman of the task force, which will also recommend ways Americans can support local officials, including police and fire departments and community health centers, in the face of a terror attack.

Mr. Bush also used the address, which was interrupted by audience applause more than 25 times, to update Americans on the status of the U.S.-led war against terrorism and lay out what steps the federal government is taking to make the United States safer.

At home, the president said the Coast Guard has taken on "expanded duties to protect our shores," the National Guard has stepped up surveillance on borders and new anti-terrorism laws give "our law-enforcement officials the necessary tools to track terrorists before they harm Americans."

Abroad, Mr. Bush said, Afghanistan's ruling Taliban made a choice not to turn over terrorist Osama bin Laden, "and now they are paying a price."

Speaking from a podium in front of a giant banner that read "United We Stand," Mr. Bush also lauded postal and health care workers — many of whom have handled the ongoing anthrax scare, as "new heroes."

"Tonight, we join in thanking a whole new group of public servants who never enlisted to fight a war, but find themselves on the front battle nonetheless: those who deliver our mail — America's postal workers," the president said, prompting cheers from the ticketed audience that included many police, firefighters, postal workers and health care workers.

"We also thank those whose quick response provided preventive treatment that has no doubt saved thousands of lives — our health care workers."

The president told Americans that they can never succumb to fear — the intent of terrorists. Despite two announcements that "credible" threats of terrorism called for "high alert" by law enforcement officials, Mr. Bush said life must go on.

"There is a difference between being alert and being intimidated," he said. "A terrorism alert is not a signal to stop your life. It is a call to be vigilant — to know that your government is on high alert, and to add your eyes and ears to our efforts to find and stop those who want to do us harm."

Throughout the speech, the president sought to uplift Americans struggling to adapt to the post-September 11 world.

"Americans have responded magnificently, with courage and with caring None of us would ever wish the evil that has been done to our country, yet we have learned that out of evil can come great good," he said.

"We are a different country than we were on September 10: sadder and less innocent; stronger and more unified; and in the face of ongoing threats, determined and courageous."

He praised schoolchildren who have sent in more than $1 million to help the children of Afghanistan. And he said Americans have fallen back on the backbone of the nation — family, religion, friends.

Mr. Bush reiterated the nature of the terrorists who killed nearly 5,000 people from 80 countries at the World Trade Center, the Pentagon and in Pennsylvania.

"We are the targets of enemies who boast they want to kill all Americans, kill all Jews, kill all Christians.

"We have seen that type of hate before, and the only possible response is to confront it and defeat it," he said to applause. "The best way to defend our homeland, the best way to make sure our children can live in peace, is to take the battle to the enemy and stop them."

When Mr. Bush first arrived in Atlanta yesterday afternoon, he toured the Centers for Disease Control and Prevention. The CDC has drawn praise for around-the-clock probing of the anthrax attacks but sharp criticism in the media for moving too slowly when the bacteria was identified in post offices.

Speaking to reporters at the center, Mr. Bush said he was open to the idea of inoculating Americans against smallpox, eradicated around the world in 1977 but feared as a possible terrorist weapon.

But he added, "One of my concerns if we were to have universal vaccination, some might lose their lives."

Mr. Bush was making the first visit by a sitting president to CDC headquarters, a complex that Sen. Max Cleland, Georgia Democrat, describes as "a shambles."

The 22 buildings scattered between two campuses just outside Atlanta have leaky roofs, with computers covered by plastic sheets to keep the rain off.

Inside some laboratories, gutters hang on chains to divert rainwater, and termite-infested floors are dangerously soft.

Last month, outdated wiring caused a 15-hour power outage that delayed the agency's identification of anthrax sent by mail to media organizations in New York.