"Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now," the president wrote in a post on Twitter. "Bob Mueller is totally conflicted, and his 17 Angry Democrats that are doing his dirty work are a disgrace to USA!"
Trump's tweet came before the trial of ex-campaign chief Paul Manafort entered its second day in federal court in Virginia. Manafort is being tried by Mueller's team.
The president's lawyer Rudy Giuliani said in a statement to NBC News Wednesday afternoon that the president "was expressing his opinion" and that "no Presidential order was issued or will be."
"The president is not obstructing, he is fighting back," White House Press Secretary Sarah Sanders said during a briefing with reporters later in the day.
Later Wednesday, ABC News, citing sources said that Trump's tweetstorm was sparked by the president learning within the past day or so that Mueller's office wants to question him about obstruction of justice. ABC said Mueller "will limit the scope of questioning and would like to ask questions both orally and written for the President to respond to."
Sessions, who endorsed Trump's presidential bid during the 2016 campaign, recused himself from the Russia investigation last March, before Mueller was appointed as special counsel to investigate both foreign interference in the presidential election, and possible collusion with Russians with the Trump campaign. The investigation is being overseen by Deputy Attorney General Rod Rosenstein, who has defended Mueller against critics in Congress.