The shock move comes days after Comey testified on Capitol Hill about the FBI's investigation into Russia's election meddling and a possible collusion between the country and Trump's campaign.
United States President Donald Trump on Wednesday fired Federal Bureau of Investigation chief James Comey, abruptly removing the top sleuth overseeing a criminal probe into whether the Trump campaign colluded with the Russian government to influence the 2016 presidential election that was won by the real estate mogul.
"You are hereby terminated and removed from the office, effective immediately," Trump said in a letter to Comey, who was four years into his 10-year term as FBI director.
Trump, while responding to a volley of questions by reporters during a surprise meeting with former secretary of state Henry Kissinger at the White House, said, "He (Comey) wasn't doing a good job. Very simply. He was not doing a good job."
The US President also asserted that the development had not affected his meeting with Russia's Foreign Minister Sergei Lavrov. Trump, however, did not respond when asked if the new FBI director will be in charge of the Russia investigation.
Earlier, Trump had explained his bombshell announcement by citing Comey's handling of the investigation by America's leading law enforcement agency into Democratic presidential nominee Hillary Clinton's use of a private email server when she was the secretary of state.
Trump told Comey, 56, that he was no longer effectively able to lead the Federal Bureau of Investigation (FBI) and it was necessary to restore "public trust and confidence" in it.
In a tweet, Trump said, "Comey lost the confidence of almost everyone in Washington, Republican and Democrat alike. When things calm down, they will be thanking me!"
The President's move comes days after Comey testified in Capitol Hill about the FBI's investigation into Russia's meddling into the US elections and a possible collusion between the Trump campaign and Moscow.
In recent days, Comey again came under fire for his handling of the investigation into Clinton's private email server. Many Democrats believe that his announcement that he was re-opening the probe 11 days before the November 8 election cost Clinton the presidency.
In the letter, Trump acknowledged that Comey had informed him "on three separate occasions" that he was not under investigation. "I nevertheless concur with the judgement of the Department of Justice that you are not able to effectively lead the bureau," he added.
Comey learned from news reports that he had been fired while addressing bureau employees in Los Angeles. It was reported that he made a joke about it to lighten the mood and called his office to get confirmation.
Comey's deputy, Andrew G. McCabe, a career FBI officer, has been appointed the acting director, the Justice Department said. The White House said the search for a director will begin immediately.
Press Secretary Sean Spicer told reporters: "The president has accepted the recommendation of the Attorney General (Jeff Sessions) and the Deputy Attorney General (Rod Rosenstein) regarding the dismissal."
In a statement released by the White House, Trump said: "The FBI is one of our nation's most cherished and respected institutions and today will mark a new beginning for our crown jewel of law enforcement."
Comey had courted controversy during the 2016 presidential polls when he revealed a renewed investigation into Hillary Clinton's email use, which the Democrats claimed had hurt her chances of becoming president.
Deputy Attorney General Rod Rosenstein, in his letter to Trump, criticised Comey's handling of the Clinton probe, including the director's decision to hold a press conference announcing its findings and releasing "derogatory information" about her.
In a separate letter, Sessions said he has concluded that a fresh start is needed at the leadership of the FBI, based on his evaluation as well the reasons expressed by the Deputy Attorney General.
"It is essential that this Department of Justice clearly reaffirm its commitment to longstanding principles that ensure the integrity and fairness of federal investigations and prosecutions," Sessions said.
Though the president has the authority to fire the FBI director for any reason, Comey is only the second director to be fired in bureau history. President Bill Clinton fired William S Sessions in 1993.
It was not immediately clear how Comey's ouster will affect the Russia probe, but Democrats said they were concerned that his ouster could derail the investigation.
"I told the president, 'Mr President, with all due respect you are making a big mistake,'" Chuck Schumer, the top Democrat in the Senate, told reporters.
The Senate Minority Leader demanded that the Department of Justice appoint a special prosecutor to investigate Russia's alleged influence on the 2016 elections.
"The firing of top officials like Sally Yates and Preet Bharara, and now Comey does not seem to be a coincidence," Schumer said.
Schumer also questioned the timing of the decision.
"If the administration had objections to the way Director Comey handled the Clinton investigation, they had those objections the minute the president got into office. But they didn't fire him then. Why did it happen today?" he asked.
House Democratic Caucus Chairman Joe Crowley said Comey's dismissal is extremely troubling.
"President Trump fired the man investigating him and his cohorts. I strongly support calls for the appointment of a special prosecutor," he said.
Terming the move as "nearly unprecedented", Indian- American Congressman Raja Krishnamoorthi said it was extremely disturbing that the Chief Executive is interfering with an ongoing investigation into his administration by firing the person charged with conducting it.
Republican Senator John McCain, the Chairman of Armed Services Committee, said he was disappointed by Trump's decision to dismiss Comey.
"James Comey is a man of honour and integrity, and he has led the FBI well in extraordinary circumstances. I have long called for a special congressional committee to investigate Russia's interference in the 2016 election," he said.
"The president's decision to remove the FBI Director only confirms the need and the urgency of such a committee," McCain said.
The New York Times noted that Comey was three years into a 10-year term, an unusually long tenure that Congress established to insulate the director from political pressure.
"Trump had already fired his acting attorney general for insubordination and his national security adviser for lying to Vice President Mike Pence about contacts with Russians. But firing Comey raises much deeper questions about the independence of the FBI and the future of its investigations under Trump", it commented.
Image: FBI Director James Comey is sworn in to testify before a Senate Judiciary Committee hearing on "Oversight of the Federal Bureau of Investigation" on Capitol Hill in Washington. Photograph: Kevin Lamarque/Reuters