President Vladimir Putin of Russia directed a vast cyberattack aimed at denying Hillary Clinton the presidency and installing Donald Trump in the Oval Office, the nation’s top intelligence agencies said in an extraordinary report they delivered today to Trump.

The officials presented their unanimous conclusions to Trump in a two-hour briefing at Trump Tower in New York that brought the leaders of the U.S. intelligence agencies face to face with their most vocal skeptic, the president-elect, who has repeatedly cast doubt on Russia’s role. The meeting came just two weeks before Trump’s inauguration as the 45th president and was underway even as the electoral votes from his victory were being formally counted in a joint session of Congress.

Soon after leaving the meeting, intelligence officials released the declassified, damning report that described the sophisticated cybercampaign as part of a continuing Russian effort to weaken the U.S. government and its democratic institutions. The report — a virtually unheard-of, real-time revelation by U.S. intelligence agencies that undermined the legitimacy of the president who is about to direct them — made the case that Trump was the favored candidate of Putin.

The Russian leader, the report said, sought to denigrate Clinton, and the report detailed what the officials had revealed to President Barack Obama a day earlier: Trump’s victory followed a complicated, multipart cyberinformation attack whose goal had evolved to help the Republican win.

The 25-page report did not conclude that Russian involvement tipped the election to Trump.

But the public report lacked the evidence that intelligence officials said was included in a classified version, which they described as information on the sources and methods used to collect the information about Putin and his associates. Those would include intercepts of conversations and the harvesting of computer data from “implants” that the United States and its allies have put in Russian computer networks.

Much of the unclassified report focused instead on an overt Kremlin propaganda campaign that would be unlikely to convince skeptics of the report’s more serious conclusions.

The report may be a political blow to Trump. But it is also a risky moment for the intelligence agencies that have become more powerful since the Sept. 11, 2001, attacks, but have had to fend off allegations that they exaggerated intelligence during the buildup to the Iraq War.

The declassified report did describe in detail the efforts of Putin and his security services, including the creation of the online Guccifer 2.0 persona and DCLeaks.com to release information gained from the hacks to the public.

“Putin and the Russian Government aspired to help President-elect Trump’s election chances when possible by discrediting Secretary Clinton and publicly contrasting her unfavorably to him,” the report by the nation’s 17 intelligence agencies concluded.

Trump, whose resistance to that very conclusion has led him to repeatedly mock the country’s intelligence services on Twitter since Election Day, issued a written statement that appeared to concede some Russian involvement. But Trump said nothing about the conclusion that Putin had sought to aid his candidacy, other than insisting that he still believes the Russian attacks had no effect on the outcome.

The president-elect’s written statement came just hours after Trump told The New York Times in an interview that the storm surrounding Russian hacking was nothing more than a “political witch hunt” carried out by his adversaries, who he said were embarrassed by their loss to him in the 2016 election. Speaking by telephone three hours before the intelligence briefing, Trump repeatedly criticized the intense focus on Russia.

“China, relatively recently, hacked 20 million government names,” he said, referring to the breach of computers at the Office of Personnel Management in late 2014 and early 2015. “How come nobody even talks about that? This is a political witch hunt.”

Later Friday evening, Vice President-elect Mike Pence told reporters that he and Trump had “appreciated the presentation” by the intelligence officials and described the conversation as “respectful.” Pence said the new administration would take aggressive action “to combat cyberattacks and protect the security of the American people from this type of intrusion in the future.”

Trump, who has consistently questioned the evidence of Russian hacking during the election, did so again Friday before he met with the intelligence officials. Asked why he thought there was so much attention on the Russian cyberattacks, the president-elect said the motivation was political.

“They are very embarrassed about it,” Trump said during an eight-minute telephone conversation.

He also repeated his criticism of the U.S. intelligence agencies, saying that “a lot of mistakes were made” in the past, noting in particular the attacks on the World Trade Center and saying, as he has repeatedly, that “weapons of mass destruction was one of the great mistakes of all time.”

But after meeting with the intelligence officials, Trump appeared to moderate his position, conceding that “Russia, China, other countries, outside groups and people are consistently trying to break through the cyberinfrastructure of our governmental institutions, businesses and organizations including the Democrat National Committee.”

The report described a broad campaign of covert operations, including “trolling” on the internet of people who were viewed as opponents of Russia’s effort. While it accused Russian intelligence agencies of obtaining and maintaining “access to elements of multiple U.S. state or local electoral boards,” it concluded — as officials have publicly — that there was no evidence of tampering with the tallying of the vote on Nov. 8.

The report, reflecting the assessments of the CIA, the FBI and the National Security Agency, stopped short of backing up Trump on his declaration that the hacking activity had no effect on the election.

“We did not make an assessment of the impact that Russian activities had on the outcome of the 2016 election,” the report concluded, saying it was beyond its responsibility to analyze U.S. “political processes” or public opinion.

The intelligence agencies also concluded “with high confidence” that Russia’s main military intelligence unit, the GRU, created a “persona” called Guccifer 2.0 and a website, DCLeaks.com, to release the emails of the Democratic National Committee and of the chairman of the Clinton campaign, John D. Podesta.

When those disclosures received what was seen as insufficient attention, the report said, the GRU “relayed material it acquired from the DNC and senior Democratic officials to WikiLeaks.” The founder of WikiLeaks, Julian Assange, has denied that Russia was the source of the emails it published.

The role of RT — the Russian English-language news organization that U.S. intelligence says is a Kremlin propaganda operation — in the Kremlin’s effort to influence the election is covered in far more detail by the report than any other aspect of the Russian campaign. An annex in the report on RT, which was first written in 2012 but not previously made public, takes up eight pages of the report’s 14-page main section.

The report’s unequivocal assessment of RT presents an awkward development for Lt. Gen. Michael T. Flynn, who is Trump’s choice to serve as national security adviser. Flynn has appeared repeatedly on RT’s news programs and in December 2015 was paid by the network to give a speech in Russia and attend its lavish anniversary party, where he sat at the elbow of Putin. Flynn has since defended his speech, insisting that RT is no different from CNN or MSNBC.

The report also stated that Russia collected data “on some Republican-affiliated targets,” but did not disclose the contents of whatever it harvested.

Intelligence officials who prepared the classified report have concluded that British intelligence was among the first to raise an alarm that Moscow had hacked into the DNC’s computer servers, and alerted their U.S. counterparts, according to two people familiar with the conclusions.

The British role, which has been closely held, is a critical part of the timeline because it suggests that some of the first tipoffs, in fall 2015, came from voice intercepts, computer traffic or informants outside the United States, as emails and other data from the DNC flowed out of the country.

“The British picked it up, and we may have had it at about the same time,” said one cyberexpert who has been briefed on the findings. British intelligence — especially the signals intelligence unit, GCHQ — takes a major role in tracking Russian activity.

The conclusions in the report were described to President Barack Obama on Thursday and to Trump on Friday by James R. Clapper Jr., director of national intelligence; John O. Brennan, director of the CIA; Adm. Michael S. Rogers, director of the National Security Agency; and James B. Comey, director of the FBI.

The key to the public report’s assessment is that Russia’s motives “evolved over the course of the campaign.” When it appeared Clinton was more likely to win, it concluded, the Russian effort focused “on undermining her future presidency,” with pro-Kremlin bloggers preparing a Twitter campaign with the hashtag #DemocracyRIP. It noted that Putin had a particular animus for Clinton because he believed she had incited protests against him in 2011.

Yet the attacks, the report said, began long before anyone could have known that Trump, considered a dark horse, would win the Republican nomination. It said the attacks began as early as July 2015, when Russian intelligence operatives first gained access to the DNC’s networks. Russia maintained that access for 11 months, until “at least June 2016,” the report concludes, leaving open the possibility that Russian cyberattackers may have had access even after the firm CrowdStrike believed that it had kicked them off the networks.

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