They were organized by Move On advocacy group, who wrote in a statement announcing the marches: "Donald Trump has installed a crony to oversee the special counsel's Trump-Russia investigation, crossing a red line set to protect the investigation".
Several hundred demonstrators gathered in New York's Times Square and chanted slogans, including "Hands off Mueller" and "Nobody's above the law" before marching downtown.
US media reported that Trump had asked White House Chief of Staff John Kelly to ask Sessions to resign, prompting Sessions to submit the letter.
Walter Shaub, who was director of the U.S. Office of Government Ethics for four years before resigning in July 2017, said the friendship between Whitaker and Clovis should disqualify Whitaker from supervising the Mueller investigation. Those TV appearances included one on CNN in which Whitaker suggested that the Mueller probe could be starved of resources. And though Democrats have called for him to recuse himself because of his past comments, the case is less clear-cut than it was for Sessions - who stepped aside because of his prominent role in Trump's campaign.
Trump has repeatedly criticized Sessions for that decision, and the Attorney General's dismissal has been expected for a long time.
"I'm profoundly concerned about it", he said. But on Wednesday, he vaulted to power when President Trump fired Attorney General Jeff Sessions and appointed Whitaker as the acting attorney general, putting him in charge of the Justice Department.
"I think this will be a very interim AG", McConnell said.
"Obey your conscience, do what is right, do what is constitutional", he said, adding: "This administration believes it is above the law and it is willing to go against the constitution".
"He is just a disgusting human being, and the fact that he did one thing that we might consider to be somewhat ethical by recusing himself from the Mueller investigation, now all of a sudden he's everybody's hero".
A Vancouver demonstration in support of special counsel Robert Mueller drew about 50 USA citizens and supporters on Thursday evening. But he also called him "smart" and "respected".
He could also refuse to sign off on a subpoena of Trump if the president refuses to voluntarily speak to investigators. Whitaker accompanied the tweet with his own comment to his followers: "Worth a Read".
"Calls for an independent counsel or commission to investigate allegations that Russian Federation tried to interfere with our elections ring hollow when similar calls for special counsels during the scandals of the Obama administration were dismissed out of hand by the same people making these demands now", Whitaker said.