Speaking to volunteers in Daytona Beach, Florida, Clinton said: "Some of you may have heard about a letter the FBI director sent" on Friday to the U.S. Congress informing it that the agency is again reviewing emails.
Comey had decided in July that the FBI was not going to seek prosecution of Clinton for her handling of classified materials on a private email server while she was secretary of state.
"It is pretty strange to put something like that out with such little information right before an election," Clinton said, adding, "It's not just strange, it's unprecedented and it's deeply troubling because voters deserve to get full and complete facts." She urged Comey to "put it all out on the table."
In tandem with Clinton, fellow Democrats on Saturday also worked to pressure Comey to provide details on a controversy that dominated the presidential campaigns on Saturday, less than two weeks before the Nov. 8 elections.
Four U.S. senators - Patrick Leahy, the longest-serving Senate Democrat, Dianne Feinstein, Thomas Carper and Benjamin Cardin - wrote Comey and U.S. Attorney General Loretta Lynch asking that they provide by Monday more detailed information about investigative steps underway.
At a press conference in Columbus, Ohio, the Congressional Black Caucus, comprised of about 45 members of the House of Representatives, nearly all Democrats, also urged Comey to be more forthcoming.
Sources close to the investigation on Friday said the latest emails were discovered as part of a separate probe into Anthony Weiner, the estranged husband of top Clinton aide Huma Abedin.
Weiner, a former Democratic U.S. congressman from New York, is the target of an FBI investigation into illicit text messages he is alleged to have sent to a 15-year-old girl in North Carolina.
Republican presidential candidate Donald Trump pounded away at the new FBI development, devoting a large part of a campaign speech in Golden, Colorado, to attacking Clinton and arguing that she is not to be trusted with the presidency.
"Her criminal action was willful, deliberate, intentional and purposeful," Trump said, standing in front of hay bales stacked in a horse barn. "Hillary set up an illegal server for the obvious purpose of shielding her illegal actions from public disclosure and exposure."
Comey, however, has not provided any details on whether the emails now under review are being seen for the first time by the FBI or the nature of their contents.
Clinton's campaign team tried to downplay the new review.
"There's no evidence of wrongdoing, no charge of wrongdoing," said John Podesta, who heads the Clinton campaign, referring to the FBI's latest announcement that it was taking "appropriate investigative steps" after learning of emails "that appear to be pertinent" to the earlier Clinton email probe.
In some of his toughest language on Saturday, Podesta portrayed Comey's letter to Congress as "light on facts, heavy on innuendo."
Clinton campaign manager Robby Mook, trying to tamp down speculation of a voter backlash this late in the campaign, said Americans had already "factored" what they knew about the email investigation into how they would cast their ballots.
"We don't see it changing the landscape" for undecided voters, Mook said.
Clinton aides also said this latest controversy has further energized her supporters.
Clinton did a campaign swing through Florida as she and Trump were thought to be in a tight race in a state famous for its role in close presidential elections.
Many analysts believe this battleground state is essential for Trump to win in order to have any chance of being elected.
In recent weeks, Trump has been running behind Clinton in most public opinion polls.
Singer and actor Jennifer Lopez was scheduled to headline a free concert in Miami on Saturday for Clinton supporters. The Clinton campaign hopes that "J.Lo," as she is known by fans, will provide celebrity star power and help Clinton connect with young voters who earlier this year flocked to Democratic primary challenger Bernie Sanders.
Justice Department officials, according to a source who asked not to be identified, were opposed to the FBI director's letter being sent to Congress and believe his actions conflict with a Justice Department memo outlining instructions that agencies should not to act in ways that could influence elections.
While Lynch did not discuss the matter directly with Comey, the source said aides were in touch with each other.
Comey let it be known he felt he had to send the letter as a follow-up to his congressional testimony earlier this year regarding the FBI's probe of Clinton's emails, the source said.
During his speech in Golden, before flying to Arizona to campaign, Trump accused the Obama administration's Justice Department of trying to protect Clinton from prosecution.
"The attorney general didn't want anything to happen to Hillary. I wonder why. It's very sad. Folks, we're living in a third world country," Trump said.