Update 8.47pm: US Attorney General Jeff Sessions has heatedly denied that he had any undisclosed meeting with the Russian ambassador or conversations with Russian officials about the US elections.
He vowed to defend his honour "against scurrilous and false allegations".
Testifying at a Senate hearing, Mr Sessions said it was a "detestable and appalling lie" to suggest that he participated in or was aware of any collusion between Russia and the Trump campaign.
In his dramatic appearance before former colleagues, Mr Sessions also contradicted a contention made by former FBI director James Comey at a hearing before the same panel last week.
Mr Comey said that, after an encounter with President Donald Trump in which he said Mr Trump pressured him to back off an investigation into the former national security adviser, Mr Comey "implored" Mr Sessions to make sure he was never left alone with the president again - but that Mr Sessions did not respond.
"He didn't recall this, but I responded to his comment by agreeing that the FBI and Department of Justice needed to be careful to follow department policy" regarding contacts with the White House, Mr Sessions said.
The former Alabama senator defended himself against accusations that he misrepresented himself during his confirmation hearing by saying he had not met with Russian officials during the campaign.
Mr Sessions argued that in the context of the hearing, "my answer was a fair and correct response to the charge as I understood it".
He said he recused himself from the Justice Department's Russia investigation only because of a regulation to require the step because of his involvement in the Trump campaign.
He never, he insisted, knew anything about the Russia probe or had any role in it.
While he had recused himself from the Russia probe, Mr Sessions said, "I did not recuse myself from defending my honour against scurrilous and false allegations".
Update 8.20pm: US Attorney General Jeff Sessions has said he did not have a third meeting with the Russian ambassador to the United States.
His impassioned response came after Senate Democrats raised questions about whether he privately met with Sergey Kislyak at an April 2016 foreign policy event at the Mayflower Hotel in Washington.
Mr Sessions said he was there for a speech by then-candidate Donald Trump, and members of Mr Sessions' staff were also in attendance.
However, he said he does not recall any private meetings or conversations with Russian officials at that event.
Mr Sessions stepped aside from the federal investigation into contacts between Russia and the presidential campaign in March after acknowledging that he had met twice with Mr Kislyak last year.
He also said the suggestion he was "aware of any collusion with the Russian government to hurt this country which I have served for 35 years, or to undermine the integrity of our democratic process, is an appalling and detestable lie".
Mr Sessions added that he has no knowledge of "any such conversations by anyone connected to the Trump campaign".
He also contradicted fired FBI director James Comey's testimony about his concerns over a meeting he had with Mr Trump.
Mr Comey testified last week that Mr Sessions did not respond when he complained that he did not want to be left alone with Mr Trump again.
This was after a February meeting in which Mr Comey said Mr Trump told Mr Sessions and others to leave the room before asking him to drop a probe into National Security Adviser Michael Flynn's contacts with Russia.
Mr Sessions said he was not silent, saying he stressed to Mr Comey the need to be careful about following appropriate policies.
He said he is confident that Mr Comey understood and would abide by the Justice Department's rules on communications with the White House about ongoing investigations.
Earlier: US Attorney General Jeff Sessions is facing sharp questions from his former Senate colleagues about his ties with Russia.
The public evidence before the Senate Intelligence committee should yield Mr Sessions' most extensive comments to date on his role in the firing of James Comey, his Russian contacts during the campaign and his decision to step aside from an investigation into possible ties between Moscow and associates of President Donald Trump.
The questions have dogged his tenure as attorney general and led him three months ago to recuse himself from the Russia probe.
Senators for weeks have demanded answers from Mr Sessions, particularly about meetings he had last summer and autumn with the Russian ambassador to the United States.
Those calls have escalated since fired FBI director Mr Comey cryptically told senators last week that the bureau had expected Mr Sessions to recuse himself weeks before he did from the investigation into contacts between Trump campaign associates and Russia during the 2016 presidential election.
Mr Sessions, a close campaign adviser to Mr Trump and the first senator to endorse him, stepped aside from the investigation in early March after acknowledging he had spoken twice in the months before the election with the Russian ambassador. He said at his January confirmation hearing that he had not met with Russians during the campaign.
Since then, questions have been raised about a possible third meeting at a Washington hotel, though the Justice Department has said that did not happen.
There had been some question as to whether the hearing would be open to the public, but the Justice Department said he requested it be because he "believes it is important for the American people to hear the truth directly from him".