The U.S. Justice Department appointed former FBI director Robert Mueller Wednesday evening as a special counsel to oversee the federal investigation into allegations that Russia and Donald Trump's campaign collaborated in order to influence the outcome of the 2016 presidential election.

It was a concession by the Trump administration, which had resisted calls from Democrats to turn the investigation over to outside counsel. The White House counsel's office was alerted only after the order appointing Mueller was signed.

The appointment gives Mueller, who led the FBI through the Sept. 11 attacks and served under presidential administrations of both parties, sweeping powers to investigate whether Trump campaign associates colluded with the Kremlin — as well as the authority to prosecute any crimes uncovered during the probe.

Earlier Wednesday, Trump complained in a commencement address that "no politician in history" has been treated worse by his foes, even as exasperated fellow Republicans slowly joined the clamour for a significant investigation into whether he tried to quash the FBI's probe by firing its director, James Comey.

Impeachment crusaders alleging obstruction of justice by Donald Trump are pointing to an apparent memo from former FBI director James Comey, which reportedly describes how the U.S. president pressured him to end a probe into Michael Flynn.

Three congressional committees, all led by Republicans, confirmed they wanted to hear from Comey, whose notes about a February meeting with the president indicate Trump urged him to drop the bureau's investigation of fired national security adviser Michael Flynn. Congressional investigators are seeking Comey's memos, as well as documents from the Justice Department related to the firing.

Many Democrats had also been calling for an independent special counsel, or prosecutor.

Representative Elijah Cummings, the top Democrat on a key House oversight panel, was among them, challenging House Speaker Paul Ryan on his previous inaction.

Cummings described Mueller as a "solid choice" before urging him "to follow the facts wherever they may lead, with integrity and independence."

Republicans frustrated

The latest political storm, coupled with the still-potent fallout from Trump's recent disclosure of classified information to Russian diplomats, overshadowed all else in the capital and beyond. Stocks fell sharply on Wall Street as investors worried that the latest turmoil in Washington could hinder Trump's pro-business agenda.

Republicans, frustrated by the president's relentless parade of problems, largely sought to cool the heated climate with assurances they would get to the bottom of scandals.

"There's clearly a lot of politics being played," Ryan said. "Our job is to get the facts and to be sober about doing that."

Putin offers record of Trump meeting

Interest was hardly limited to the U.S.

No less a commentator than Russia's Vladimir Putin called the dramatic charges swirling around Trump evidence of "political schizophrenia spreading in the U.S." He offered to furnish a "record" of the Trump-diplomats meeting in the Oval Office if the White House desired it.

There was no word on what that record might entail, a question many were likely to raise in light of Trump's recent warning to Comey that he had "better hope" there were no tapes of a discussion they'd had.

During a commencement address at the U.S. Coast Guard Academy in New London, Conn., on Wednesday, Donald Trump said about himself: 'No politician in history, and I say this with great surety, has been treated worse or more unfairly.' (Susan Walsh/Associated Press)

The White House disputed Comey's account of the February conversation concerning Flynn, but did not offer specifics. Several members of Congress said that if Trump did suggest that Comey "let this go" regarding Flynn's Russian contacts, it was probably just a joke or light banter.

White House aides mostly kept a rare low profile, avoiding going on television. Trump did not offer any commentary on Twitter and did not directly address the controversies during a commencement address at the Coast Guard Academy, though he delivered a broadside against the forces he sees as working against him.

Trump says he's worst-treated politician 'ever'

"No politician in history, and I say this with great surety, has been treated worse or more unfairly," he said. "You can't let the critics and the naysayers get in the way of your dreams.... I guess that's why we won. Adversity makes you stronger. Don't give in, don't back down.... And the more righteous your fight, the more opposition that you will face."

Trump speaks with Russia's Foreign Minister Sergei Lavrov, left, and Russian ambassador Sergei Kislyak, right, on May 10 — the day after James Comey was fired. (EPA)

Questions about Trump's conduct have been mounting for weeks, most recently with two explosive revelations — that in February the president pressed Comey to drop a federal investigation into Flynn's contacts with Russia and that he disclosed classified information to the senior Russian officials last week.

Both allegations came from anonymous sources, and the White House was quick to denounce the leaks and deny any impropriety, insisting the president never tried to squelch the Flynn investigation nor did he make inappropriate disclosures to the Russians.

Trump himself issued a statement Wednesday night saying much the same thing.

"As I have stated many times, a thorough investigation will confirm what we already know — there was no collusion between my campaign and any foreign entity," the president said in response to Mueller's appointment. "I look forward to this matter concluding quickly."

Putin says critics 'just dumb'

Putin, watching from afar, said the "evolving political struggle" had gone from something of an amusement to serious cause for concern, and he suggested Trump's critics were stoking anti-Russian sentiment to damage the president.

"These people either don't understand that they are hurting their own country, and in that case they are just dumb," Putin said. "Or they do understand everything, and that means that they are dangerous and unscrupulous."

On Capitol Hill, Comey was clearly the man in demand, with three committees working to seat him at their witness tables: the House oversight committee, the Senate intelligence committee and the Senate judiciary committee.