Russian president Vladimir Putin ordered a hidden campaign to influence America's presidential election in favour of Donald Trump over Hillary Clinton, US intelligence agencies have declared.
It is the government's first formal allegation in support of the claims of interference that Mr Trump and his supporters have resisted.
The intelligence report, an unclassified version of a more-detailed classified account given earlier to Mr Trump, the White House and congressional leaders, withheld any evidence to back up its assertions.
The president-elect said after his own meeting with the nation's top intelligence officials that it was clear Russian email hacking did not deliver him the presidency.
There was no suggestion that Russia affected actual vote counting or tampered with ballot machines, said the unclassified version, which was the most explicit account to date of Russian efforts to interfere with the US political process.
But the report said Russia's actions included hacking into the email accounts of the Democratic National Committee and individual Democrats including Mrs Clinton's campaign chairman, John Podesta, and using state-funded propaganda and paid "trolls" to make comments on social media services.
The unclassified version included footnotes acknowledging that it "does not include the full supporting information on key elements of the influence campaign".
But it said its conclusions were identical to the classified version, which was more detailed.
The report said the Russian effort was both political and personal.
"Russia's goals were to undermine public faith in the US democratic process, denigrate Secretary Clinton and harm her electability and potential presidency," it said.
"We further assess Putin and the Russian government developed a clear preference for President-elect Trump."
Mr Putin most likely wanted to discredit Mrs Clinton because he blames her for inciting mass protests against his regime in late 2011 and early 2012, and because he resents her for disparaging comments she has made about him, the report said. It said the Russian effort was the "boldest yet" intended to affect a US election.
As Mr Trump met in New York with intelligence officials for his briefing about Russia's campaign, Congress tallied the Electoral College votes, officially confirming Mr Trump's November victory.
Mr Trump has been dismissive of the intelligence agencies' claims of Russia's involvement for months, long before he saw the classified information on Friday.
After finally seeing the intelligence behind the claims of the outgoing Obama administration, he released a one-page statement that did not address whether Russia sought to meddle.
Instead, he said, "there was absolutely no effect on the outcome of the election".
The top US intelligence official told Congress on Thursday that intelligence agencies had no way of gauging what influence this meddling had on the outcome of the election. It was unclear on Friday what evidence Mr Trump had to support his claims.
According to the public report, when it appeared that Mrs Clinton might well win the election, Mr Putin turned his focus to undermining her presidency.
Mr Trump acknowledged in his statement that "Russia, China, other countries, outside groups and people" are consistently trying to hack US networks, including the Democratic National Committee's.
He said, as did the intelligence report, that "there was no tampering whatsoever with voting machines".
Mr Trump said that as president he would appoint a team to develop a plan to "aggressively combat and stop cyberattacks".
Before seeing the classified intelligence report on Friday, Mr Trump dismissed the assessment of interference and told The New York Times the focus on Russia's involvement is a "political witch hunt" by adversaries who are embarrassed they lost the election.
"They got beaten very badly in the election," Mr Trump said. "They are very embarrassed about it. To some extent, it's a witch hunt. They just focus on this."
The officials in Friday's meeting with Trump in New York - director of National Intelligence James Clapper, CIA director John Brennan and FBI director James Comey - also briefed senior politicians on Friday morning. Mr Obama received his classified briefing on Thursday.
A senior law enforcement official said the FBI in its investigation had repeatedly stressed to Democratic National Committee officials the importance of obtaining direct access to servers "only to be rebuffed until well after the initial compromise had been mitigated".
The official said the FBI had to rely on a "third party" for information, but eventually got access to the material it needed.