Comey's handling of the investigation into Hillary Clinton's emails was central to the dismissal, the administration said. In a memo circulated by the White House before Comey's dismissal, Deputy Attorney General Rod Rosenstein criticized Comey's handling of the Clinton probe, including the director's decision to hold a news conference announcing its findings and releasing "derogatory information" about Clinton.
In a tweet, hedge fund billionaire Ray Dalio — who worked with Comey at Bridgewater Associates — called Comey a "hero."
Tweet: I would not feel good about myself if I did not make clear that I have come to know and admire James Comey as a man of integrity and a hero
Edward Snowden, a whistleblower who is accused of breaking espionage laws to reveal NSA data collection, called for condemnation of Comey's firing.
Tweet: This FBI Director has sought for years to jail me on account of my political activities. If I can oppose his firing, so can you.
Hillary Clinton's running mate, Sen. Tim Kaine, D-Va., tweeted that the dismissal shows that the administration is "frightened" of the investigation into Russia's ties to presidential election interference.
Sen. Bob Corker, R-Tenn., chairman of the Senate Foreign Relations committee who has been among the more critical GOP senators when it comes to some of Trump's policies, said the removal of Comey "will raise questions."
Nobel laureate economist Paul Krugman questioned whether the timing was tied to the testimony of former acting Attorney General Sally Yates, who spoke this week about the alleged Russian interference in the 2016 election.
Tweet: And surely the strong bet is that Russia/Flynn etc. is about to close in. Nixon: "I am not a crook". Trump: "I am not a traitor" Uh huh 3/
Sen. Lindsey Graham, R-S.C., a former presidential candidate who ran against Trump and is a member of the Senate judiciary committee, said he supported a "fresh start" at the FBI.
Tweet: Given the recent controversies surrounding the director, I believe a fresh start will serve the FBI and the nation well
Frank Quattrone, a banker who was once investigated by then-U.S. Attorney Comey, also weighed in.
Tweet: The man who once lied about me on national TV and wrongfully indicted me just got fired. Karma's a powerful force.
Sen. Mark Warner, D-Va., a former venture capitalist and tech entrepreneur and current member of the Senate Select Committee on Intelligence, called the shakeup "deeply troubling."
— With reporting by CNBC's Christine Wang, NBC News and The Associated Press