"Must recuse!" as they rallied in Times Square, according to video of the protest. "It's not ok for someone to break the law and be above the law".
Organizers called on people around the United States to protest at 5 p.m.
By Thursday morning, more than 1,000 people said on a Facebook page for the event that they planned to attend, and more than 3,200 said they were interested.
Thousands marched in cities across the country, including New York City and Washington, D.C.
Democratic Socialist Alexandria Ocasio-Cortez Wins Election In a Landslide
Women in general had a record-breaking night in the House, with more than 90 of them expected to join the House's freshman class. Ocasio-Cortez's district, New York's 14th, has a foreign-born population that is now nearing almost 50 percent.
Crowds also turned out in Chicago; Greensboro, North Carolina; Chattanooga, Tennessee, and many other places.
Answering allegations that the White House had used a clip edited by a notorious right-wing conspiracy theorist, Trump spokeswoman Sarah Sanders said: "We stand by our statement".
The sacking followed months of Mr Trump criticising Mr Sessions, mainly for his decision to step aside from the Russian Federation inquiry in March 2017.
"It is imperative that the Administration not impede the Mueller investigation", she tweeted.
'Nor has a President ever before appointed an acting attorney general who is on record describing the means by which the President could curtail an ongoing investigation of the President's conduct'.
The investigation has so far produced 32 criminal charges and guilty pleas from four former Trump aides.
Florida teenager charged with strangling, burying mother
At first he drove to Daytona's Holly Hill area to dump her corpse but headed back to find a shovel before heading to the church. He called the suspect a "soulless individual", and probably one of the "top three sociopaths" he ever crossed paths with.
The protests kicked off Thursday night as media outlets began recirculating a 2017 interview Whitaker gave on "The David Webb Show" in which he argued that a US president has the power to end any federal investigation without obstructing justice.
As part of Whitaker's new role as the head official at the Justice Department, he will oversee the Russian Federation probe and the agency's other federal investigations, including the NY prosecutors' look into the finances of Trump and his former aides.
"We're a co-equal branch in the government, and we have a responsibility to understand what happened in 2016 and to prevent it from ever happening again", King said. He has been interviewed as part of Mueller's probe. He said "there was so much smoke" surrounding Clinton that it made flawless sense for Trump Jr.to take a meeting at which he expected to receive dirt on her. "You would have somebody from your campaign take the meeting to try to get the information", he said. Whitaker is poised to take over the Russian Federation investigation, a probe he has criticized in the past.
"I thought we had made progress", Wilson said. "I think the president understands that it's best for Bob Mueller to finish his investigation". Others believe it could be a first step towards canning the Mueller probe before Democrats gets their teeth into the matter when the new Congress convenes in January; The FBI Special Counsel is believed to be wrapping up his investigation and ready to hand over a report (to the attorney general) that may not reflect well on the President and his associates.
Republican Sen. Susan Collins also voiced concern about the move, tweeting on Wednesday that "Special Counsel Mueller must be allowed to complete his work without interference - regardless of who is AG".
Ayanna Pressley is officially Massachusetts’s first black congresswoman
Ocasio-Cortez, a socialist, had previously worked on Bernie Sanders' campaign for the Democratic presidential nomination. Polis, who declared victory late Tuesday night, has always been outspoken in favour of marijuana legalisation.