After spending weeks accusing the World Health Organization of helping the Chinese government cover up the early days of the coronavirus epidemic in China, President Trump said on Friday that the United States would terminate its relationship with the agency.

“The world is now suffering as a result of the malfeasance of the Chinese government,” Mr. Trump said in a speech in the Rose Garden. “Countless lives have been taken, and profound economic hardship has been inflicted all around the globe.”

In his 10-minute address, Mr. Trump took no responsibility for the deaths of 100,000 Americans from the virus, instead saying China had “instigated a global pandemic.”

There is no evidence that the W.H.O. or the government in Beijing hid the extent of the epidemic in China, and public health experts generally view Mr. Trump’s charges as a way to deflect attention from his administration’s own bungled attempts to respond to the virus’s spread in the United States.

A spokeswoman for the W.H.O. in Geneva, where word of Mr. Trump’s announcement first landed at 9 p.m., said the agency would not have a response until Saturday.

In April, when he was asked about Mr. Trump’s accusation that the W.H.O. was “China-centric,” Tedros Adhanom Ghebreyesus, the organization’s director-general said: “It is wrong to be any ‘country-centric.’ I am sure we are not China-centric. The truth is, if we are going to be blamed, it is right to blame us for being U.S.-centric.”

Public health experts in the United States reacted to Mr. Trump’s announcement with alarm.

“We helped create the W.H.O.,” said Dr. Thomas Frieden, the former director of the Centers for Disease Control and Prevention, which has worked with the organization since its creation in 1948.

“We’re part of it — it is part of the world,” Dr. Frieden said. “Turning our back on the W.H.O. makes us and the world less safe.”

The Infectious Diseases Society of America “stands strongly against President Trump’s decision,” said Dr. Thomas M. File, its president. “We will not succeed against this pandemic, or any future outbreak, unless we stand together, share information and coordinate actions.”

It is not clear whether the president can simply withdraw the United States from the World Health Organization without Congressional approval.

“The president can’t unilaterally withdraw us,” said Lawrence O. Gostin, director of the World Health Organization Collaborating Center on National & Global Health Law at The Georgetown University Law Center.

“It’s a nonstarter,” he added. “This is literally a whim of one man, without any consultation with Congress, in the middle of the greatest health emergency of our lifetime.”

The reaction among Democrats in Congress was swift and negative.

Representative Ami Bera, Democrat of California and the chairman of the House Foreign Affairs Subcommittee on Asia, the Pacific, and Nonproliferation, called Mr. Trump’s announcement “shameful and irresponsible.”

The W.H.O. “is not a perfect organization,” he said on Twitter, “but leaving will make the United States and the world less safe. President Trump is ceding American global leadership and handing it over on a golden platter to China.”

Representative Nita Lowey, Democrat of New York and the chairwoman of the House Appropriations Committee, said: “The president wants to blame everyone else — the W.H.O., Twitter, the media — when his own shortcomings as a leader are contributing to harm and further dividing us here at home and among global partners.”

The administration’s response to the emergency has been fumbling and inadequate, many public health experts say, especially when compared to China’s.

The coronavirus has been the leading cause of death in the United States since mid-April, killing roughly 100,000 citizens to date. By comparison, only 4,600 Chinese citizens have died of the infection.

About 20,000 Americans are infected each day, while China virtually ended its outbreak by April. On most days China records zero to five new infections, usually in travelers from abroad.

The W.H.O. was founded in 1948 as part of the postwar creation of the United Nations and is the world’s premier global health organization. Mr. Trump supported and generously funded the organization as it fought an Ebola outbreak in Africa for three years, but abruptly turned on the W.H.O. a few weeks ago, when he began accusing the organization of doing too little to warn the world of the spread of the coronavirus.

In fact, the agency issued its first alarm on Jan. 4, just five days after the local health department of Wuhan — at the time a city few non-Chinese had even heard of — announced a cluster of 27 cases of an unusual pneumonia at a local seafood market.

The W.H.O. followed up with a detailed report the next day. On Jan. 20 and 21, a W.H.O. field team visited China and reported that there could be human-to-human transmission of the new pneumonia-causing virus.

Almost simultaneously, China’s leading epidemiologist, who had just completed his own investigation on behalf of the Beijing government, confirmed during a Jan. 20 interview on state television that transmission to doctors was occurring in Wuhan, although he said on a recent interview with CNN that local officials had lied about it and even tried to mislead him.

Within three days, Beijing had shut off all travel out of Wuhan. Mr. Trump did not order any restrictions on travel from China until Jan. 31.

The United States has been by far the W.H.O.’s largest donor since its inception. The budget for the W.H.O. is about $6 billion, which comes from member countries around the world. In 2019, the last year for which figures were available, the United States contributed about $553 million.

The American government and private donors, including the Bill and Melinda Gates Foundation, Bloomberg Philanthropies and Rotary International, have wielded enormous influence on W.H.O. policies.

For example, although the war on smallpox that was begun in the 1960s was at first largely a Soviet initiative, the W.H.O. chose American doctors, including Dr. William Foege and Dr. Donald A. Henderson, to lead the global campaign.

The agency also chose American-made vaccines over Soviet ones for the war on polio.

For many years, the American government, working on behalf of the Western pharmaceutical industry, pressured the W.H.O. not to publicly fight for lower drug prices that might threaten the patent monopolies of American companies.

That changed in the early 2000s, when many American companies began sub-licensing their patents and technology to generics makers in India and elsewhere.

No American has ever been director-general of the W.H.O., but that is because of a decades-old understanding that the World Bank and the United Nations Children’s Fund would always be run by Americans, while the leadership of some other U.N. agencies, including the W.H.O., would be taken in turn by other nations.

The C.D.C. and many other branches of the American government have worked with the W.H.O. for decades. Along with the C.D.C., doctors from the American military and even the Army’s 82nd Airborne Division worked in cooperation with the W.H.O. to fight the 2014 West African Ebola epidemic — partly in an effort to keep it from reaching the United States.

The W.H.O. provides essential diplomatic cover when American government agencies work in foreign countries. All countries that belong to the U.N. are also de facto members of the ruling body of the W.H.O.

Asked for comment Friday, a C.D.C. official said the agency did not know what impact the announcement would have on its 72-year-old working relationship with the W.H.O., and referred all further questions to the White House.

As he began facing harsh questions about his handling of the disease here, Mr. Trump swiftly diverted the blame to the W.H.O., threatening in a letter earlier this month to pull funding if it did not “commit to major substantive improvements in the next 30 days.”

In fact, under Dr. Tedros, the agency has been in the middle of major reforms for several years, focusing more of its attention on pandemics and less on the causes championed by wealthy donors, including tobacco, lung cancer and obesity.

Last month at the World Health Assembly — the annual meeting of the health ministers of all U.N. member nations that serves as the agency’s governing board — other member states rebuffed Mr. Trump’s demands. They voted instead to conduct an “impartial, independent” examination of the W.H.O.’s pandemic response.

Mr. Trump’s Rose Garden address came as cities across the United States were convulsing with protests over recent cases of police brutality against black Americans.

He did not take questions after delivering his speech, even as assembled reporters shouted for him to address protests in Minneapolis.