Senate Democratic leader Chuck Schumer said he spoke to Trump and told him he was "making a very big mistake" in firing Comey, adding the president did not "really answer" in response.
An independent investigation into Moscow's role in the election "is now the only way to go to restore the American people's faith", Schumer said.
Though many Democrats have criticised Comey's handling of the Clinton email probe, they said they were troubled by the timing of Trump's firing of him.
Senator Richard Burr, the Republican chairman of the Senate Intelligence Committee, which is overseeing its own investigation into Russian interference during the election, said he was also troubled by the timing of Comey's termination. "His dismissal, I believe, is a loss for the Bureau and the nation," Burr said.
Intelligence agencies concluded in a January report that Russian President Vladimir Putin had ordered an effort to disrupt the 2016 election, with the aim of helping Trump.
CNN reported yesterday that federal prosecutors had issued grand jury subpoenas to former Trump national security adviser Michael Flynn, seeking business records, as part of the probe into Russian interference in the election.
Trump's firing of Comey came a day after former acting Attorney General Sally Yates told a Senate panel that she had informed the White House on January 26 that Flynn was at risk of blackmail by Moscow because he had been untruthful about his discussions with the Russian ambassador, Sergei Kislyak. Trump fired Flynn 18 days later.
Russia has repeatedly denied any meddling in the election and the Trump Administration denies allegations of collusion with Russia.
Trump, in a letter to Comey released by the White House, said: "It is essential that we find new leadership for the FBI that restores public trust and confidence in its vital law enforcement mission." The president told Comey in the letter that he accepted the recommendation of Attorney General Jeff Sessions that he could no longer provide effective leadership.
Sessions advised Trump's campaign before being picked by the President to lead the Justice Department. Sessions had recused himself from involvement in the Russia investigation, after he misstated his own 2016 contacts with Russia's ambassador to Washington.
Comey's deputy, Andrew McCabe, became acting FBI director.
Pushing back against critics of the move, White House officials said Deputy Attorney General Rod Rosenstein, a career prosecutor who took office on April 25, assessed the situation at the FBI and concluded that Comey had lost his confidence.
The White House released a memo in which Rosenstein wrote: "I cannot defend the Director's handling of the conclusion of the investigation of Secretary Clinton's emails, and I do not understand his refusal to accept the nearly universal judgment that he was mistaken."
Comey, 56, had been the target of criticism from many quarters for his handling of a probe involving Clinton's use of a private email server while she was US Secretary of State under Obama. As recently as yesterday, the FBI clarified remarks that Comey made on the matter last week.
The firing came as a shock to FBI staff, nearly all of whom had confidence in Comey despite the controversy surrounding his handling of the Clinton email situation, according to an FBI official who spoke on condition of anonymity. The official said there was concern among agents that the firing was a political act related to the Russian investigation.
Other current and former US intelligence and law enforcement officials questioned the White House explanation for Comey's firing.
"Trump praised him for the work on the email investigation, so that's not it," said Austin Berglas, a former FBI supervisory agent on hacking cases. "I think he realised the extent of the Russia investigation under way and moved him out. To me, that's the only logical explanation right now."
Trump's dismissal of Comey does not mean the FBI's investigation into Russia's interference in the U.S. presidential election will be disrupted or end - career FBI staffers can continue the probe even as the search for a new FBI director begins, legal experts said.
