The Reagan administration today offered to buy out all 2,400 residents of Times Beach, Mo., after confirming that the town is too contaminated with the toxic compound dioxin to be safe for human habitation.

Environmental Protection Agency Administrator Anne M. Burford said she decided to buy the entire town late last night after learning that dioxin levels were 300 times what the federal Centers for Disease Control in Atlanta considers safe.

Hundreds of Times Beach residents alternately cheered Burford's move at a hotel just outside the poisoned hamlet and scorned it as a publicity stunt designed to defuse the controversy surrounding the EPA.

Times Beach became a national symbol of the growing public health threat posed by hazardous wastes after federal officials learned in December that virtually the entire one-square-mile town was contaminated with dangerous levels of dioxin.

The compound, in tiny doses, has caused cancer, nervous disorders and birth defects in laboratory animals. Its effect on humans is being studied, although it is believed to be potentially lethal.

"The president . . . cares about you deeply, and I'm delighted to be here today to share this good news with you," Burford said before a crowd of reporters in a locked briefing room as the Times Beach residents listened outside on loudspeakers.

The residents were not allowed to speak to Burford, who was married on Sunday and changed her name from Gorsuch. She left town immediately after her brief announcement.

Laine Jumper, a Times Beach resident who agitated for months for the buy-out, said he was relieved at the prospect of federal help but embittered by the fact that the government knew about the dioxin problem for more than a decade and that the EPA had known for six weeks that the contamination was dangerous.

"A lot of people think Times Beach is full of backwater rubes," said Jumper, "but we can smell a political deal as easily as someone in Washington."

The buy-out is expected to cost $33 million, Burford said, with the money coming from the $1.6 billion "Superfund" established to clean up the nation's most dangerous hazardous waste dumps. Missouri will be required to chip in $3.3 million more, she said.

This is the first time the Superfund has been used to relocate victims of toxic waste exposure, Burford said. The plans for the buy-out are still rough, but the Federal Emergency Management Agency and state officials said they will begin immediately to assess the property of all Times Beach residents.

They said they will make offers based on the real estate value of the homes and businesses before the dioxin contamination was discovered.

The decision means that Times Beach probably will cease to exist. The buy-out program, which is to begin in 60 to 90 days, is not compulsory, but residents of the town said they doubt that anyone will want to stay behind in an abandoned area where the dioxin contamination is so high.

Burford also announced that her agency has added Times Beach to the list of 418 contaminated areas around the country that are priorities for being cleaned up under the Superfund program. The cleanup project, expected to cost tens of millions of dollars, will not begin until all residents have had a chance to sell their homes and businesses, she said.

The residents of Times Beach were forced to leave their homes in early December by flood waters that rose as high as 42 feet. On Dec. 23 the CDC advised residents not to go home, after preliminary studies of the town's soil showed dioxin levels as high as 100 parts per billion. (The CDC ranks any concentration of dioxin above 1 part per billion as a serious health risk. One part per billion is comparable to 1 second in 31.7 years.)

Times Beach is one of at least 100 communities in Missouri where dioxin mixed with waste oils was sprayed on dirt roads in the 1970s to keep down dust. Its dangers were unknown then to many government officials.

Burford and Rita M. Lavelle, EPA's ousted chief of hazardous waste control programs, said until recently that they needed more data to know whether the health dangers at Times Beach warranted the use of Superfund money to buy out the town.

While the EPA deliberated, all but about 100 men, women and children had moved to temporary shelter with aid from the Federal Emergency Management Agency. Almost all town enterprises had shut down, including four churches, the local liquor store, and several garages.

EPA officials were also debating the public relations value of various cleanup options for the area. In a memo to Lavelle, EPA's dioxin specialist said removing enough dirt to leave only .05 parts per billion "is likely to be very expensive." A second cleanup option, to a level of 1 part per billion "allows immediate action for the agency, and good press." The third option--to clean up the site leaving 100 parts per billion--"could create extreme political backlash. Public and press scrutiny will be substantial."

In the meantime, Burford said she needed to know whether the poison, which was believed concentrated around the town's roads, had moved into homes and yards during the floods. The latest data indicated it had not, she said. Burford said she based her decision on a review of additional data by the CDC, which documented contamination levels of up to 300 parts per billion.

It took until last night, Burford said, "with my staff working like dogs over the weekend and the holiday," to go over the CDC findings and to reach her decision.

"Suppose I had waltzed out here without any test results and ordered relief for Times Beach," she said. "You know what the allegations would be. It would have been malfeasance, and rightfully."

But for the people of Times Beach such explanations are almost beside the point. Federal and state officials knew as early as 1973 that dioxin had been sprayed throughout the state, and as early as 1975 that it was considered a threat to human health. Yet it was not until last spring that the EPA began moving to study the statewide extent of the problem, and not until November that testing began in Times Beach.

At that time the EPA was playing down the seriousness of the contamination in Missouri in what agency critics contend was an attempt to safeguard the Senate seat of Republican John C. Danforth, then locked in a reelection battle against Democrat Harriett Woods.

Danforth, who won the race narrowly, had complained to the EPA administrator in late October that the agency's handling of the matter "is not good enough."

On Nov. 1, a day before the election, the EPA dispatched a last-minute news release announcing a "promising" new technique to decontaminate dioxin.

Several agency officials later acknowledged that the release was "cooked up" to help the Danforth campaign and that the technology was still in the test-tube stage.

"You have to wonder if anybody in Washington really cares," said Evelyn Zuffalt, who drove a school bus in Times Beach and raised seven children there. "This buy-out answers our immediate problem, but I don't think we'll have answers for generations. I think of my children. It will be a stigma all their lives. It'll be on their insurance policies that they lived there, that they grew up there."

Jumper, Zuffalt and others called the buy-out project "only a beginning." They said the EPA and the state should compensate them for the disruption to their lives and for all future health problems. However, 16,102 Vietnam veterans exposed to dioxin through Agent Orange have sought for years without success to obtain federal compensation for health problems that they blame on the toxin.

Government officials said they will not know for months, maybe years, what will become of the land that is now Times Beach. Fencing it off will not prevent health hazards, since future floods could carry the poison to other communities, they said. The Superfund clean-up program may include excavating much of the dirt in Times Beach and incinerating it, they said.

"The science really hasn't caught up with the problem," said Ron Kucera, deputy director of the state natural resources department. "That was the problem with dioxin from the beginning.