"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.
Sessions took the lead in imposing Trump's travel ban targeting a number of mostly Muslim nations, canceling the so-called Dreamer program that let the children of undocumented immigrants remain in the US and forcing the separation of migrant families as part of a border crossing crackdown. The rally was one of many across the country in reaction to Trump's appointing Whitaker.
Demonstrators in New York City chanted slogans including "Hands off Mueller" and "Nobody's above the law" before marching downtown. It was an older crowd, and many held signs deriding Trump or supporting Mueller.
The probe has been criticised by Mr Sessions's successor Matthew Whitaker.
That's raising questions about the fate of Mueller's investigation.
The investigation until now has been overseen by Deputy Attorney General Rod Rosenstein, who appointed Mueller past year and granted him fairly broad authority. Rosenstein was at the center of a recent controversy after it was reported that in the spring of 2017 he raised the idea of Cabinet members invoking the 25th amendment to remove the president and suggested that he should secretly record Trump.
Ted Cruz fends off Beto O'Rourke in Texas Senate race
He set fundraising records and made Cruz work for the victory, but Cruz ultimately pulled out a three-point victory. A Texas Democrat hasn't won any of the almost 30 statewide offices since 1994, the country's longest losing streak.
Trump blamed the recusal for the appointment of Mueller, who took over the Russian Federation investigation two months later and began examining whether Trump's hectoring of Sessions was part of a broader effort to obstruct the probe.
"The (Democrats') first step would be to protect the investigation, they would do that by making sure all documents were secured by ensuring Mueller's investigative team was not broken up", he said, adding: "On Capitol Hill, the Democrats are quite likely to call for a floor vote immediately to countermand and reverse the firing". Mueller and Trump's lawyers have negotiated for months about a possible interview, with no agreement in sight. He has said that Hillary Clinton should have been prosecuted for her email scandal as secretary of state and that Trump made the right call in firing FBI Director James Comey. "You would have somebody from your campaign take the meeting to try to get the information", he said.
"The chances of Mueller being interfered with goes up dramatically" with Whitaker's appointment, he said. "And Congress must act to protect the Mueller investigation", another organizer said.
Those include an opinion piece on CNN.com in which he said Mueller would be going too far, and straying beyond his mandate, if he were to investigate Trump's family finances.
The Justice Department has the ultimate authority over Mueller's investigation into Russia's alleged interference in the 2016 presidential election and interactions between Trump associates and Russian officials.
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". Whitaker accompanied the tweet with his own comment to his followers: "Worth a Read".
The requirement that Whitaker notify Congress if he blocks Mueller from doing something is "about political accountability, not legal accountability", Rosenzweig said.