"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.
Trump has attacked Sessions several times for his recusal. In June, the president wrote in a post on Twitter that Mueller was continuing to investigate "all because Jeff Sessions didn't tell me he was going to recuse himself."
"I would have quickly picked someone else. So much time and money wasted, so many lives ruined...and Sessions knew better than most that there was No Collusion," Trump tweeted.
The White House did not respond to a request to elaborate on the president's remarks. The office of the special counsel and the Department of Justice declined to comment.
Senate Minority Leader Chuck Schumer, D-N.Y., called the president's tweet "just another attempt to make the American people look at his latest shiny object."
In a post on Twitter Wednesday, Schumer quoted the president's tweet and wrote: "The Mueller-Rosenstein investigation is making progress at record speed: 35 indictments, 5 guilty pleas & Trump's campaign chairman on trial."
California congressman Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, called the president's tweet "an attempt to obstruct justice hiding in plain sight."
AshLee Strong, spokesperson for House Speaker Paul Ryan, R-Wisc., told NBC News that Ryan "has said many times the investigation should continue to its conclusion." His position has not changed, she said.
The special counsel is investigating the president's tweets criticizing Sessions as part of a wider inquiry into whether the president has obstructed justice, The New York Times reported last month.
Trump and his legal team have escalated their attacks on the special counsel in recent weeks.
Giuliani made headlines on Monday saying that "collusion is not a crime." The president doubled down on his attorney's comments, tweeting on Tuesday that "Collusion is not a crime, but that doesn't matter because there was No Collusion (except by Crooked Hillary and the Democrats)!"
It's true that "collusion" is not a word that appears in any criminal statute that Mueller is examining. Mueller, however, has pursued charges involving conspiracy, obstruction of justice and making false statements.