Donald Trump claims he is the victim of a "witch hunt" after a former FBI director was appointed to lead an investigation into his campaign's alleged ties to Russia.
"This is the single greatest witch hunt of a politician in American history!" the US president tweeted.
The Justice Department on Thursday appointed former FBI Director Robert Mueller as a special counsel to lead a federal investigation into allegations Trump's campaign collaborated with Russia to sway the 2016 election that put him in the White House.
It comes after Trump complained to graduates at the US Coast Guard Academy that "no politician in history ... has been treated worse or more unfairly" than him.
The surprise announcement to hand the probe over to Mueller, a lawman with deep bipartisan respect, was a striking shift for Trump's Justice Department, which had resisted increasingly loud calls from Democrats for an outside prosecutor.
It immediately escalated the legal stakes - and the potential political damage - for a president who has tried to dismiss the matter as partisan witch hunt and a "hoax."
Mueller will have sweeping powers and the authority to prosecute any crimes he uncovers.
In a second Twitter post, Trump said: "With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special councel (sic) appointed"
Trump later deleted the tweet and republished it spelling counsel correctly.
The announcement, the latest in the shock-a-day Washington saga, was made by deputy Attorney General Rod Rosenstein.
The White House counsel's office was alerted only after the order appointing Mueller was signed, said a senior White House official.
In a written statement, Trump insisted there were no nefarious ties between his campaign and Russia.
"A thorough investigation will confirm what we already know - there was no collusion between my campaign and any foreign entity," he declared. "I look forward to this matter concluding quickly."
Mueller's broad mandate gives him not only oversight of the Russia probe, but also "any matters that arose or may arise directly from the investigation."
That would surely include Trump's firing last week of FBI Director James Comey.
Mueller, a former federal prosecutor at the Justice Department, was confirmed as FBI director days before the September 11, 2001, attacks that would ultimately shape his tenure.
The FBI's counterterror mission was elevated in those years, as the US intelligence agencies adjusted to better position America to prevent another attack of such magnitude.
He was so valued that President Barack Obama asked him to stay on two years longer than his 10-year term. Comey succeeded him, appointed by Obama.
Mueller was appointed on Wednesday by Rosenstein, who had faced criticism as the author of a memo that preceded Comey's firing.
Rosenstein said the appointment was "necessary in order for the American people to have full confidence in the outcome."
Republicans have largely stood behind Trump in the first months of his presidency as the FBI and congressional investigations into Russia's election meddling intensified.
But Republican politicians have grown increasingly anxious since Trump fired Comey, who had been leading the bureau's probe - and after Comey associates said he had notes from a meeting in which Trump asked him to shut down the investigation into the Russia ties of former National Security Adviser Michael Flynn.
First reactions from Congress were mainly positive. House Speaker Paul Ryan said the appointment was consistent with his goal of ensuring that "thorough and independent investigations are allowed to follow the facts wherever they may lead."
Republican Jason Chaffetz of Utah, chairman of the House Oversight Committee, said Mueller was a "great selection. Impeccable credentials. Should be widely accepted." And not a moment too soon, Democrats said.
"I believe Mueller will be independent, he will be thorough and he will be fair and he's not going to be easily swayed," said Elijah Cummings of Maryland, top Democrat on the oversight panel.
Flynn "won't honour subpoena"
A lawyer for fired General Flynn has informed the Senate Intelligence Committee he will not honour its subpoena for private documents.
That's according to the panel's chairman, Republican Senator Richard Burr of North Carolina.
Senator Burr said the panel's members are not surprised, adding: "We'll figure out on General Flynn what the next step, if any, is."
The committee is one of several on Capitol Hill investigating possible collusion between Russia and President Donald Trump's 2016 campaign.
General Flynn was ousted earlier this year from his senior administration job.
Trump "weighs downsizing Spicer's role"
Trump is reportedly considering scaling back White House press secretary Sean Spicer's public role amid a broader shake-up of his communications team.
Officials told Politico that Spicer is no longer expected to do a daily, on-camera briefing after Trump's foreign trip.
The US leader has reportedly told his allies and aides he doesn't want Spicer explaining his policies anymore, with officials saying deputy press secretary Sarah Huckabee Sanders will likely appear at the podium more often.
Officials also said Trump is generally fed up with his communications team and its response to his administration's recent scandals.
A White House official and an outside adviser said Trump on Monday blasted communications director Mike Dubke because of the stream of negative headlines about his presidency.