US President Donald Trump has fired the director of the FBI over his handling of the inquiry into Hillary Clinton's emails, the administration says.
The White House shocked Washington by announcing that James Comey "has been terminated and removed from office".
But Democrats said he was fired because the FBI was investigating alleged links between the Trump campaign and Russia.
The move came as it emerged Mr Comey gave inaccurate information to Congress last week about Mrs Clinton's emails.
An FBI investigation into her use of a private email server to store sensitive information while secretary of state remains closed. Mr Comey said in July last year it was careless but not criminal.
The White House said the search for a successor for Mr Comey would begin immediately. In a tweet on Wednesday morning, Mr Trump said Mr Comey would be replaced "by someone who will do a far better job, bringing back the spirit and prestige of the FBI".
It is only the second time the head of the FBI has been fired.
Why was James Comey fired?
President Trump wrote in a letter to Mr Comey that he agreed with US Attorney General Jeff Sessions' recommendation that "you are not able to effectively lead the Bureau".
Mr Sessions said the department of justice was "committed to a high level of discipline, integrity, and the rule of law", and "a fresh start is needed".
Many have expressed surprise that Mr Comey should be fired for his handling of the investigation into Mrs Clinton's emails, given that Mr Trump once praised the FBI director's conduct in the matter.
In the final days of the presidential campaign, Mr Trump told a rally it "took guts" for Mr Comey to reopen the inquiry. "What he did brought back his reputation," Mr Trump said.
But on Tuesday, Deputy Attorney General Rod Rosenstein said he "cannot defend the director's handling of the conclusion of the investigation of Secretary [of State Hillary] Clinton's emails, and I do not understand his refusal to accept the nearly universal judgement that he was mistaken".
"Almost everyone agrees the director made serious mistakes; it is one of the few issues that unites people of diverse perspectives."
Mr Rosenstein also said:
- Mr Comey had been wrong to "usurp" the previous attorney general in July 2016 when he announced the Clinton emails inquiry should be closed without prosecution
- That he had compounded his error by "gratuitously" releasing "derogatory information" about Mrs Clinton
Mr Comey was addressing FBI agents in Los Angeles when, according to US media, he learned he had just been fired when he saw the news on television.
The 56-year-old - who was three-and-a-half years into his 10-year term as FBI director - reportedly laughed, thinking it was a prank.
What about the Russia investigation?
Democrats swiftly suggested that Mr Trump had fired Mr Comey to influence the FBI inquiry into whether members of the Trump election campaign colluded with the Kremlin.
The House of Representatives and Senate intelligence committees are looking into the same allegations but no conclusions have yet been reached.
"Were these investigations getting too close to home for the president?" Senate Minority Leader Chuck Schumer asked a Tuesday evening press conference.
"This does not seem to be a coincidence," he added.
Mr Trump responded on Twitter that Mr Schumer had recently expressed his lack of confidence in the FBI chief.
President Trump has repeatedly insisted the Russia allegations are "fake news". He and his Secretary of State Rex Tillerson will meet Russian Foreign Minister Sergei Lavrov in Washington later on Wednesday.
What are Republicans saying?
- "I am troubled by the timing and reasoning of Jim Comey's termination." - Richard Burr, the head of the Senate Intelligence Committee
- "My staff and I are reviewing legislation to establish an independent commission on Russia." - Justin Amash, a conservative Michigan congressman, adding that a line in Mr Trump's letter, that Mr Comey had informed him three times he was not under investigation, was "just bizarre"
- Nebraska Senator Ben Sasse, a longstanding Trump critic, said the "timing of the firing is very troubling"
- "I've spent the last several hours trying to find an acceptable rationale for the timing of Comey's firing," Senator Jeff Flake, Republican of Arizona, said on Twitter. "I just can't do it."
A cover-up? Anthony Zurcher, BBC News, Washington
Donald Trump and senior justice department officials are framing the firing of James Comey as a result of his botched investigation into Hillary Clinton's email server. They do so using language that even Clinton backers would probably support.
Democrats, to put it bluntly, aren't buying it, however - not from this White House. They are dismissing this Clinton explanation as a smokescreen, and view the suddenness of the move as an attempt to subvert the ongoing FBI investigation into possible ties between Russia and the Trump campaign.
At the very least, their calls for an independent investigation into the matter will become deafening - and some Republicans may now be inclined to agree.
How did Comey handle the email inquiry?
Mr Comey has been criticised by Democrats for the handling of his investigation into whether Mrs Clinton's use of a private email server when secretary of state compromised national security.
The now-former FBI director made two interventions during the 2016 election campaign to make pronouncements about the investigation.
He said in July the case should be closed without prosecution, but then declared - 11 days before November's election - that he had reopened the inquiry because of a discovery of a new trove of Clinton-related emails.
He told the Senate last week it had made him "mildly nauseous" to think his intervention could have affected the election, but insisted he would make the same decision again. Mrs Clinton lays part of the blame for her shock election defeat last November on Mr Comey.
He told the Senate Judiciary Committee on 3 May that Mrs Clinton's top aide, Huma Abedin, had forwarded "hundreds and thousands" of emails, "some of which contain classified information", to her husband.
But the FBI conceded on Tuesday that Ms Abedin had sent only two email chains containing classified information to her husband, Anthony Weiner, for printing.
In June 1972, five men were arrested trying to bug the offices of the Democratic National Committee (DNC) at the Watergate office and hotel complex in Washington. The break-in was traced to President Richard Nixon's supporters.
The firing of James Comey is drawing comparisons with the so-called Saturday Night Massacre of 1973, when Mr Nixon fired an independent special prosecutor investigating the break-in and the subsequent cover-up.
"Not since Watergate has a president dismissed the person leading an investigation bearing on him," the New York Times wrote late on Tuesday.
Mr Nixon later admitted he had been aware of the cover-up and had tried to halt the FBI's inquiry. He later became the only US president to resign.