Russian President Vladimir Putin sought to help Donald Trump win the presidential election, a US intelligence report says.
The unclassified report says the Russian leader "ordered" a campaign aimed at influencing the election.
Moscow has not commented, but Russia has previously denied the claims.
After being briefed on the findings, Mr Trump stopped short of accusing Russia of interfering, saying only that the election outcome was not affected.
Mr Trump later went on to blame "gross negligence" by the Democratic National Committee for "allowing" the hacking to take place.
Following the report's findings, the US Department of Homeland Security announced that voting machines and other election databases would be classified as "critical infrastructure" and given more protection from cyber-attacks.
The 25-page report says that the Kremlin developed a "clear preference" for Mr Trump.
Russia's goals, the document added, were to "undermine public faith" in the US democratic process and "denigrate" his Democrat opponent Hillary Clinton, harming her electability and potential presidency.
"We assess Russian President Vladimir Putin ordered an influence campaign in 2016 aimed at the US presidential election," it said.
The unclassified version contained no detailed evidence of Mr Putin's alleged role, but it said Russia's actions included:
- Hacking into the email accounts of the Democratic National Committee and top Democrats;
- Using intermediaries such as WikiLeaks, DCLeaks.com and Guccifer 2.0 persona to release the information acquired from the hackings;
- Using state-funded propaganda and paying social media users or "trolls" to make nasty comments
The report says Mr Putin liked Mr Trump because he had vowed to work with Russia and the Russian leader had had "many positive experiences working with Western political leaders whose business interests made them more disposed to deal with Russia, such as former Italian Prime Minister Silvio Berlusconi and former German Chancellor Gerhard Schroeder".
In contrast, Mr Putin had blamed Mrs Clinton for inciting anti-government protests in 2011 and early 2012 "and because he holds a grudge for comments he almost certainly saw as disparaging him".
The identities of the Russian agents allegedly directly responsible for the hack are known to US authorities but have not been released publicly, reports say, citing intelligence sources.
The document, a cut-down version of the classified report presented to President Barack Obama on Thursday, was released shortly after intelligence chiefs briefed Mr Trump on their findings.
Since winning the election on 8 November, Mr Trump has repeatedly questioned US intelligence claims of Russian hacking. In a statement after the briefing, the president-elect declined to single out Russia, but said he had "tremendous respect for the work and service done" by those in the US intelligence community.
"While Russia, China, other countries, outside groups and people are consistently trying to break through the cyber infrastructure of our governmental institutions, businesses and organisations including the Democrat National Committee, there was absolutely no effect on the outcome of the election," he said.
"Whether it is our government, organisations, associations or businesses we need to aggressively combat and stop cyber-attacks. I will appoint a team to give me a plan within 90 days of taking office."
What Trump didn't say: Nick Bryant, BBC News, New York
It's an explosive report claiming the Russian government launched a massive, multi-faceted operation to sway the outcome of the US presidential election. After being briefed on the findings, Mr Trump issued a statement saying that the outcome of the election was not affected.
Tellingly he did not endorse the report's central finding that the Kremlin was behind the cyber campaign.
In his statement, he also expressed tremendous respect for US intelligence agencies. But in the lead-up to the meeting he was publicly scornful of American spies, in a manner completely unprecedented for a president-elect.
Earlier, he protested that the Russian hacking allegations were part of a political witch-hunt by political adversaries badly beaten, as he put it, in the election. He clearly thinks they're being used by his opponents to question the legitimacy of his presidency.
The meeting at Trump Tower involved National Intelligence Director Gen James Clapper, CIA Director John Brennan and FBI Director James Comey.
Mr Trump, who will be inaugurated on 20 January, described their talks as "constructive".
US officials previously said the Kremlin was behind the hacking of Democratic Party emails. Russia denies the allegations, and WikiLeaks founder Julian Assange said Moscow was not the source for the site's mass leak of emails from the Democratic Party.
Shortly before his briefing with intelligence chiefs, Mr Trump dismissed the Russian hacking claims as a "political witch-hunt" by his opponents. He told the New York Times that US institutions had been the target of previous hacking attacks, but they had not received the same media attention.
Last week President Obama ordered the expulsion of 35 Russian diplomats from the US over the alleged hacking. Russia has said it will not reciprocate.
Russian reaction - where's the evidence? By BBC Monitoring
The Kremlin has yet to react to the report but state media rubbished it.
"The headline-grabbing accusations are based on TV programmes, posts on social networks and material from entertainment publications," Russia's most-watched TV station Channel One said.
Official state Rossiya 1 TV viewed the report through the prism of future relations between the two countries.
Its Washington correspondent Alexander Khristenko said the report was an "attempt to undermine the president-elect's legitimacy".
"Donald Trump himself remained critically-minded about the intelligence services' conclusions," he said, adding that "this is clearly not the sort of reaction from Trump that Washington hawks were counting on."