Former special counsel Robert Mueller said in dramatic US congressional testimony he had not exonerated President Donald Trump of obstruction of justice, and defended the integrity of his inquiry under repeated attacks by conservative Republican allies of the President.

Key points: Democrats had hoped the hearings would weaken Donald Trump's re-election prospects

Democrats had hoped the hearings would weaken Donald Trump's re-election prospects Robert Mueller is expected to be asked about the conclusions to his 448-page report

Robert Mueller is expected to be asked about the conclusions to his 448-page report The former special counsel has been advised not to go beyond the findings outlined in his May report

Mr Mueller, 74, initially testified he would have sought to indict Mr Trump were it not for a Justice Department policy against charging a sitting president.

But hours later he corrected himself and said "we did not reach a determination as to whether the President committed a crime".

The former investigator was surrounded by news photographers as he took his place in the packed hearing room, showing little apparent emotion as he scanned the scene.

He faced a series of rapid-fire questions from politicians in both parties, several times asking them to repeat their questions and often referring them to the text of the report itself.

Mr Mueller avoided being drawn into arguments with Republicans who hammered away at his inquiry, often frustrating them with responses such as, "I am not going to get into that".

The former FBI director, who spent 22 months investigating what he concluded was Russian interference in a "sweeping and systematic fashion" in the 2016 US election and Mr Trump's conduct as a candidate, appeared for more than three-and-a-half hours before the House of Representatives' Judiciary Committee.

Mr Mueller's investigative report said the inquiry found insufficient evidence to establish that Mr Trump and his campaign engaged in a criminal conspiracy with Russia.

'It is not a witch hunt', says Mueller

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 9 minutes 35 seconds 9 m Robert Mueller made a rare appearance to comment on the investigation into Russia's election interference last month.

The Judiciary Committee's Democratic chairman, Jerrold Nadler, praised Mr Mueller and said no-one, including Mr Trump, was "above the law".

The Intelligence Committee's Democratic chairman, Adam Schiff, accused Mr Trump's 2016 campaign of "disloyalty to country" for inviting, encouraging and making full use of Russian election meddling.

But the President's Republican allies on the committees tried to paint Mr Mueller's investigation as unfair, with Louie Gohmert accusing him of having "perpetuated injustice" and conservative congressman Guy Reschenthaler calling the manner in which the inquiry was conducted "un-American".

"Welcome, everyone, to the last gasp of the Russian collusion conspiracy theory," said Devin Nunes, the Intelligence Committee's top Republican.

The 448-page report, released in redacted form on April 18, did not reach a conclusion on whether Mr Trump committed the crime of obstruction of justice in a series of actions aimed at impeding the inquiry, but did not exonerate him.

Mr Trump has claimed that the Mueller inquiry resulted in the President's "complete and total exoneration".

"Did you actually totally exonerate the President?" Mr Nadler asked.

"No," Mr Mueller replied.

Mr Trump, following the hearing on Twitter, vehemently disagreed and claimed it was the "greatest witch hunt in US history".

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"Why didn't Robert Mueller & his band of 18 Angry Democrats spend any time investigating Crooked Hillary Clinton, Lyin' & Leakin' James Comey, Lisa Page and her Psycho lover, Peter S, Andy McCabe, the beautiful Ohr family, Fusion GPS, and many more, including HIMSELF & Andrew W?," Mr Trump wrote.

After the hearing, Democratic Representative Ted Lieu told reporters:

"What we established today in the hearing is that we have a felon sitting in the White House. Donald Trump committed multiple crimes of obstruction of justice."

Mr Mueller, accused by Mr Trump of heading a "witch hunt" and trying to orchestrate a "coup" against the Republican President, said his inquiry was conducted in "a fair and independent manner" and that members of the special counsel's team "were of the highest integrity".

"It is not a witch hunt," Mr Mueller added.

Mr Trump has accused Mr Mueller of having conflicts of interest, including saying Mr Mueller wanted the President to appoint him as FBI director after firing James Comey, which Mr Mueller denied.

'Obstruction of justice strikes at the core of government'

Mr Mueller warned he would not stray beyond the conclusions in his 448-page report. ( Reuters: Leah Millis )

Democrats focused on five actions by Mr Trump that Mr Mueller had investigated as potential obstruction of justice, including at one point telling his White House counsel to remove the special counsel.

"Obstruction of justice strikes at the core of the Government's efforts to find the truth and to hold wrongdoers accountable," Mr Mueller testified.

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Under questioning by Mr Nadler, he acknowledged that the report detailed "multiple acts by the President that were capable of exerting undue influence over law enforcement investigations, including the Russian interference and obstruction investigations."

"Well, the finding indicates … that the President was not exculpated for the acts that he allegedly committed," Mr Mueller said.

Attorney-General William Barr, appointed by Mr Trump, subsequently cleared the President of obstruction of justice after receiving Mr Mueller's report.

But in his opening statement to the Judiciary Committee, Mr Mueller reiterated that his team had decided not to make a determination on the question of obstruction.

"Based on Justice Department policy and principles of fairness, we decided we would not make a determination as to whether the President committed a crime. That was our decision then and remains our decision today," Mr Mueller said.

He did say that a president could be charged with a crime after leaving office.

He also said he would not talk about the "impeachment issue".

Republicans fuel Trump's accusations of a witch-hunt

Mr Mueller re-iterated his report's point that Mr Trump may have committed an obstruction of justice. ( AP: Andrew Harnik )

Republican congressman John Ratcliffe accused Mr Mueller of exceeding his authority in the report's extensive discussion of potential obstruction of justice by Mr Trump after the special counsel made the decision not to draw a conclusion on whether the President committed a crime.

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While Mr Ratcliffe agreed that Mr Trump was not above the law, he said the President should not be "below the law" either.

Another Republican, Steve Chabot, said Wednesday's hearing was the "last, best hope" by Democrats "to build up some sort of groundswell across America to impeach President Trump. That's what this is really all about today."

He also told Mr Mueller some people thought his report was "a pretty one-sided attack on the President".

Mr Mueller's investigation led to criminal charges against 34 people and three Russian entities.

People who were convicted at trial or pleaded guilty included Mr Trump's former campaign chairman, deputy chairman and other aides.

Mr Mueller then appeared before the House Intelligence Committee for more questioning.

Democrats control the House, while Mr Trump's fellow Republicans control the Senate.

ABC/wires