Donald Trump has fired FBI Director James Comey during the bureau's investigation into whether the tycoon's campaign had links to Russia's meddling in the presidential election.
The sudden sacking of the nation's most senior law enforcement official outraged Democrats who said the White House was interfering in the inquiry.
In a letter to Mr Comey, Mr Trump said the firing was necessary to restore "public trust and confidence" in the FBI.
Mr Comey has come under intense scrutiny in recent months for his public comments on an investigation into Democrat Hillary Clinton's emails, including two letters he sent to Congress on the matter in the closing days of last year's campaign.
Mr Trump made no mention of Mr Comey's role in the Clinton investigation, which she has blamed in part for the election result.
But the White House circulated a scathing memo by Deputy Attorney General Rod Rosenstein, criticising Ms Comey's handling of the Clinton investigation, including the director's decision to hold a news conference announcing its findings and releasing "derogatory information" about Mrs Clinton.
Since Attorney General Jeff Sessions recused himself from the bureau's Trump-Russia probe, Mr Rosenstein, his deputy, has been in charge.
This is only the second firing of an FBI director in history. President Bill Clinton dismissed William Sessions amid allegations of ethical lapses in 1993.
Democrats criticised Mr Trump's action, comparing it to President Richard Nixon's "Saturday Night Massacre" decision to fire the independent special prosecutor overseeing the Watergate investigation in 1973.
"This is Nixonian," Senator Bob Casey declared on Twitter.
Senator Ron Wyden called it outrageous and called for Mr Comey to immediately be summoned to testify to Congress about the status of the Trump-Russia investigation.
Adam Schiff of California, the senior Democrat on the House intelligence committee, said the White House was "brazenly interfering" in the investigation.
Republican Senator John McCain of Arizona said Congress must form a special committee to investigate Russia's interference in the election.
Democrats expressed deep scepticism about the stated reasons for Tuesday's firing, raising the prospect of a White House effort to hinder the inquiries by the FBI and congressional panels.
Mr Trump will now appoint Mr Comey's successor, and the White House said the search for a replacement was beginning immediately.
Mr Trump has ridiculed the investigations as a "hoax" and has denied that his campaign was involved in Russia's meddling.
In his letter to Mr Comey, he asserted that the FBI Director had informed him "on three separate occasions that I am not under investigation".
The stunning announcement came shortly after the FBI corrected aspects of Mr Comey's sworn testimony on Capitol Hill last week.
Mr Comey told politicians that Huma Abedin, an aide to Mrs Clinton, had sent "hundreds and thousands" of emails to her husband's laptop, including some with classified information.
On Tuesday, the FBI told the Senate Judiciary Committee that only "a small number" of the thousands of emails found on the laptop had been forwarded there while most had simply been backed up from electronic devices.
Most of the email chains on the laptop containing classified information were not the result of forwarding, the FBI said.
Some politicians did welcome news of the dismissal.
"Given the recent controversies surrounding the director, I believe a fresh start will serve the FBI and the nation well," said Republican Lindsay Graham, chairman of a Senate Judiciary subcommittee investigating the Russian campaign interference.
Mr Comey, 56, was nominated by President Barack Obama for the FBI post in 2013 to a 10-year term.
Praised frequently by both parties for his independence and integrity, Mr Comey has spent three decades in law enforcement.
But his prominent role in the 2016 presidential campaign raised questions about his judgment and impartiality.
Though the FBI did not recommend charges against Mrs Clinton for mishandling classified information, Mr Comey was critical of her decision to use a personal email account and private internet server during her four years as secretary of state.
Mr Comey strongly defended his decisions during the hearing last week. He said he was "mildly nauseous" at the thought of having swayed the election but also said he would do the same again.
Mrs Clinton has partially blamed her loss on Mr Comey's disclosure to Congress less than two weeks before Election Day that the email investigation would be revisited.
Mr Comey later said the FBI, again, had found no reason to bring any charges.