The American Red Cross said it was undertaking one of the largest emergency operations in its history, and federal disaster-relief teams were descending on stricken areas across the Gulf Coast. Private volunteer groups were preparing to deliver hot meals to hundreds of thousands of residents made refugees by the storm and the flooding caused by it.

Even before the storm hit the Gulf Coast, the Federal Emergency Management Agency had positioned 23 disaster medical assistance teams and seven search-and-rescue teams around the region. It also delivered generators, tarps and stockpiles of water, ice and ready-to-eat meals, agency officials said.

FEMA also sent two teams of veterinarians to provide care to any injured pets or other animals.

More than 5,000 National Guard troops were called up over the weekend to assist in relief operations, despite the burden of providing troops to support operations in Iraq and Afghanistan. Tens of thousands more may be called as the extent of the damage becomes clear, officials said. National Guard officials said many of the troops mobilized for storm duty had recently returned from overseas combat zones.

Some 3,500 Louisiana National Guard troops were on active duty to help with housing, security, power generation, food distribution and debris removal, with 6,500 more available. Mississippi called up 850 National Guard troops, and Alabama mobilized more than 1,500. Officials said that nearly 100,000 additional members of the Guard from the Southeast were available on short notice.

The Alabama contingent includes military police, engineers and Special Forces troops who have all recently served in Iraq or Afghanistan, according to Lt. Col. Robert Horton of the Alabama National Guard.

''We are prepared to respond to any natural disaster and to support the war on terrorism,'' Horton said. ''We just have to deal with both missions.''

The National Guard units were being joined by thousands of public workers and private volunteers in one of the largest emergency mobilization efforts in recent years.

The American Red Cross had opened 239 shelters by Monday evening and sent 166 emergency response vehicles and thousands of volunteers to the stricken area, according to a Red Cross spokeswoman, Renita Hosler.

Hosler said the Red Cross response to the storm would probably surpass its efforts last year when four hurricanes ripped across Florida in quick succession. The agency spent roughly $130 million on hurricane relief programs last year, she said.

Along with the Red Cross, the Southern Baptist Disaster Relief Fund and Spirit of America, a nonprofit humanitarian group, moved in mobile kitchens and had prepared more than 500,000 hot meals for evacuees by Monday evening, officials said.

President Bush over the weekend declared an emergency for Louisiana, Alabama and Mississippi, allowing federal agencies to prepare to respond.

If necessary, FEMA has 500 truckloads of ice, 500 of water and 350 of meals available to distribute over the next 10 days, an official said.

By Monday afternoon, Michael D. Brown, the agency director, was trying to discourage fire and emergency service teams from areas outside of the storm zone from sending crews unless they were specifically asked by local or federal authorities.

''Such efforts must be coordinated so that fire-rescue efforts are the most effective possible,'' Brown said in a statement.

Mark Smith, the chief information officer for the Louisiana Office of Homeland Security and Emergency Preparedness, urged the hundreds of thousands of people who fled southeastern Louisiana ahead of the storm to stay where they were.

Smith said late Monday that power and communications had been cut off in much of the affected region and that state officials had not had a chance to assess damage or to rescue people trapped in more remote areas of the state.

''We're going to be launching rescue missions to the flooded portions to do search and rescues, and those efforts will be ongoing throughout the night,'' he said. ''By nature, Louisiana has more water than anywhere you can conceive of - and that was before today.''