WASHINGTON — A federal emergency response plan for vaccinating the entire U.S. population against a smallpox attack envisions recruiting at least 1.3 million volunteers to staff health clinics 16 hours a day for a full week.

The mass vaccination guidelines issued Monday outline an unprecedented medical challenge that public health experts say is hard to imagine: inoculating 288 million Americans quickly and calmly against the backdrop of a bioterrorist attack. Never in the nation's history has such a rapid, large-scale inoculation program been undertaken, although drafters of the guidelines said they drew on the lessons of smaller vaccination campaigns.

While federal officials have released little information on the potential threat of a bioterrorist attack, the Bush administration says it wants the country to be ready to respond. Officials said they would treat even a single case of smallpox as a terrorist incident and move quickly, with the helps of states, to nationwide vaccination.

"The purpose of this plan is to take the next step in getting states ready in the event of an attack," said Walter Orenstein, director of the National Immunization Program at the Centers for Disease Control and Prevention. "The goal here is to help states and localities develop the capacity to provide vaccine to very large numbers of people as rapidly as possible."

In the 50-page document sent to state and local health commissioners, the CDC lays out a step-by-step scenario for dealing with smallpox — from ordering refrigerators for storing vaccine to scheduling daily trash pickup at dozens of vaccination clinics.

It urges states to identify and train personnel to not only administer the vaccine but also handle security, transport people, brief the media, direct traffic, run instructional videos, collect medical histories, enter data into computers and respond to other emergencies.

"To do mass vaccination in 10 days would be a total nightmare," said Donald Leung, editor of the Journal of Allergy and Clinical Immunology.

The plan instructs state officials to identify 20 sites for vaccinating every 1 million people. High schools, health clinics, arenas, theaters, conference halls and perhaps shopping malls could be used, according to the plan.

To meet a goal of vaccinating 1 million in seven to 10 days requires a minimum of 4,680 people, according to the plan.

"I'm astounded at the number of people it takes to actually make this happen and the work it will entail," said Maryland Secretary of Health Georges Benjamin.

In his state of 5 million, Benjamin calculated he would need to identify at least 23,500 people to operate 20 vaccination clinics — and that, he said, assumes a well-rehearsed plan that is implemented smoothly. He said the "idealized guidelines" drafted by federal officials do not even take into account the hurdles and delays of routine vaccination.

Elderly people bundled in several layers of clothing in a wheelchair, residents who don't speak English or have a complicated medical history all make the process more involved, he said.

"It's better that we get this now than later," said Benjamin, who is also president of the Association of State and Territorial Health Officers. "But it's going to take a lot of work to make this happen."

The federal government has purchased $428 million worth of vaccine — enough to vaccinate every American. The Department of Health and Human Services has begun distributing grants to states to begin planning how they would respond to a bioterrorism attack.

As the Bush administration continues to expand counterterrorism capabilities and contemplates war with Iraq, smallpox has emerged as perhaps the most feared biological weapon because it is so contagious and deadly. Routine vaccination stopped in this country in 1971 and the disease was declared eradicated worldwide a decade later.

Yet the prospect that a hostile state such as Iraq or North Korea may have acquired the deadly virus has sparked fresh interest in returning to the vaccine. Federal health officials are developing two strategies simultane-ously — one to inoculate tens of thousands of health care workers who would likely be the first to treat an outbreak, and Monday's blueprint for responding to an attack with a rapid nationwide vaccination program.

Public-health leaders welcomed the guidelines but said they were struck by the sheer enormity of the task.

The CDC is awaiting a decision by President Bush on how many people to vaccinate immediately. Despite an early recommendation to inoculate just 20,000 medical personnel, sources say the White House is studying a proposal to vaccinate 500,000 first responders. The decision on early vaccination is complicated by the fact the vaccine itself can cause sometimes severe reactions.