The US attorney general has been accused of "waging a media campaign" for President Donald Trump ahead of the Mueller report's long-awaited release.
The 400-page report is the result of an investigation into alleged Russian interference during the 2016 election.
William Barr is holding a news conference before it goes public.
US Senate Democratic leader Chuck Schumer has said there is now a "crisis of confidence" over Mr Barr's impartiality and independence.
During his Thursday press conference, Mr Barr reiterated points made in an earlier summary in which he said Mr Mueller's report cleared Mr Trump of any collusion.
However, he clarified Mr Mueller did not go as far as to completely exonerate the president of obstruction of justice.
Both Mr Trump's supporters and detractors are now eagerly awaiting the full - albeit redacted - report's release.
It will be sent to Congress between 11:00 and midday local time (15:00 GMT and 16:00 GMT). It will be published online sometime after this.
Speaking at a press conference before the release, Mr Barr described redactions to the report as "limited".
He also confirmed Mr Trump's personal lawyers got a copy of the redacted report earlier this week and said members of congressional committees from both parties would be given almost completely unredacted copies to review.
What is the Mueller report?
The report contains the findings of a 22-month investigation into possible collusion between Russia and the Trump presidential campaign back in 2016.
It was led by Special Counsel Robert Mueller, who was chosen to run the investigation in 2017 following concerns from US intelligence agencies that Russia had tried to tip the election in Mr Trump's favour.
He also looked into whether Mr Trump obstructed justice when he asked for the inquiry into former national security adviser Michael Flynn to end, and later fired FBI chief James Comey.
Mr Flynn has since pleaded guilty to lying to the FBI about his contacts with Russia - one of six former Trump aides and 30 other people, including 12 Russians, charged in connection with the investigation.
What do we know already?
So far all the public have seen of the report is the four-page summary released by US Attorney General Barr.
It contained Mr Mueller's main conclusions. The first, that Mr Trump did not collude with Russia during the 2016 campaign, and the second, that he did not completely exonerate him of the charge of obstructing justice.
He said: "While this report does not conclude that the President committed a crime, it also does not exonerate him."
Exactly what this means is what many hope to discover with the release of the report on Thursday.
But it may not be that easy to ascertain. The report has been redacted, with a colour-code indicating the reasons why.
As a result, according to the BBC's North America editor Jon Sopel, "it might look more like a colouring book than a report".
How has President Trump reacted?
Mr Trump and his supporters immediately jumped on the fact that no collusion was found.
"After three years of lies and smears and slander, the Russia hoax is finally dead. The collusion delusion is over," the president told a cheering crowd in Grand Rapids, Michigan, last month.
The Republican president has repeatedly described the investigation as "a witch hunt".
However, he has not addressed the fact that the report does not completely clear him of the allegation of obstructing justice.
Ahead of the release on Thursday the president posted a flurry of tweets, in which he described the investigation as the "greatest political hoax of all time" and said it constituted "harassment".
PRESIDENTIAL HARASSMENT!— Donald J. Trump (@realDonaldTrump) April 18, 2019
What do his opponents say?
Leading Democrats have called for the Mueller report to be published in full, and pledged to make use of the party's majority control of committees in the House of Representatives to continue investigating the president.
They have also raised concerns over Mr Barr's handling of the report since Mr Mueller's team handed it to the Justice Department.
A joint statement, signed by the Democratic chairs of five House committees, said the press conference was "unnecessary and inappropriate, and appears designed to shape public perceptions of the report before anyone can read it".
"Rather than letting the facts of the report speak for themselves, the Attorney General has taken unprecedented steps to spin Mueller's nearly two-year investigation," Mr Nadler, who is the House Judiciary Committee chairman, added while speaking to reporters on Wednesday.
Mr Schumer has said the "American people deserve to hear the truth" and has called on Mr Mueller to testify personally.