Donald Trump feared the probe into Russian election meddling would doom his presidency and attempted to fire Special Counsel Robert Mueller, the top investigator said in his long-awaited report, which was finally released on Thursday.

Shortly after the Justice Department made public the more than 400-page report in redacted form, Trump declared a political victory, saying with a smile: "I'm having a good day."

The report — which landed as the United States dives into the ferment of a bitter presidential campaign — did support Trump's repeated assertions that he never colluded with Russian intelligence efforts to tilt the 2016 election in his favour.

"As I have been saying all along, NO COLLUSION - NO OBSTRUCTION!" he tweeted.

However, the report — based on nearly two years of interviews by Mueller's team with Trump's inner circle — emphasised that despite the Republican president's claim, he had not been cleared of obstruction of justice.

And while finding that no Americans took part in the Russian meddling, Mueller determined that Trump was happy enough to gain an advantage from the dirty tricks.

This included the release by WikiLeaks of emails stolen by Russian agents from the team of Trump's Democratic opponent, Hillary Clinton.

"The campaign expected it would benefit electorally from information stolen and released through Russian efforts," the report said.

'This is the end of my presidency'

The report released on Thursday paints an unflattering picture of Trump in crisis mode as the scandal first hit the White House in 2017.

US intelligence services had already been looking into the Russian meddling. But after Trump fired his FBI director, James Comey, Mueller was appointed as an independent prosecutor to handle the highly sensitive probe.

"The president slumped back in his chair and said: 'Oh my God. This is terrible. This is the end of my presidency. I'm f---ed'," the Mueller report said.

Top Trump advisor Kellyanne Conway rejected that portrayal, telling reporters: "That was not the reaction of the president that day."

Obstruction of justice?

Trump has been proven right regarding collusion, Mueller says, but a second allegation — that he might have attempted to obstruct justice by derailing the probe — remains far from defused.

Attorney General William Barr made a nationally televised statement just before the report was released to give the administration's spin on the results.

He explained Trump's hostile behaviour during the drama as the actions of a president who was "frustrated and angered".

"And at the same time, the president took no act that in fact deprived the special counsel of the documents and witnesses necessary to complete his investigation," Barr said.

Mueller's detailing of Trump's actions, however, leaves a murkier record.

Trump told his then White House counsel, Don McGahn, to tell the acting attorney general that Mueller "must be removed" — something McGahn refused to do.

And that, Mueller reports, was a pattern.

"The president's efforts to influence the investigation were mostly unsuccessful, but that is largely because the persons who surrounded the president declined to carry out orders or accede to his requests," the report said.

In a passage likely to be welcomed by Trump's Democratic opponents in Congress, Mueller stated that it could be up to lawmakers to determine whether Trump obstructed justice.

"If we had confidence after a thorough investigation of the facts that the president clearly did not commit obstruction of justice, we would so state," his report said.

"Based on the facts and the applicable legal standards, however, we are unable to reach that judgment."

Legislators have "the authority to prohibit a president's corrupt use of his authority", it said.

Fight not finished

The secretive Mueller probe has consumed Washington — and the first half of Trump's first term in office — over the last two years.

Throughout, Trump has labelled the investigation a "witch hunt" while his Democratic opponents have talked up the extraordinary idea that an American president might have been colluding with Russian agents.

Publication of the report — minus parts blacked out for legal or security reasons — will allow everyone to get a more complete picture of a scandal that has been tangled in conspiracy theories.

But given the charged political atmosphere in Washington and the left-right chasm across the country as Trump dives into his 2020 re-election bid, the debate over what really happened is likely to rage on.

Trump is supremely confident that his opponents have failed in what he claims was high-level "treason" to destroy his presidency.

Claiming that Barr has been working to whitewash the Russia report, Democrats will push for more details and testimony in the near future — including from Mueller himself.

"It is clear Congress and the American people must hear from Special Counsel Robert Mueller in person to better understand his findings," Nadler, who heads the House judiciary committee, tweeted.

Nadler set a deadline of May 23 for Mueller to come to Capitol Hill.