U.S. President Donald Trump's intervention into Roger Stone criminal case connected to his own conduct drew fierce rebukes Saturday from Democrats and a few Republicans, with calls for investigations and legislation.

Trump on Saturday painted Stone as a victim and lashed out against Democratic presumptive presidential nominee Joe Biden and former President Barack Obama, with whom Biden served as vice president.

"Roger Stone was targeted by an illegal Witch Hunt that never should have taken place. It is the other side that are criminals, including the fact that Biden and Obama illegally spied on my campaign - AND GOT CAUGHT!" the president tweeted.

Trump's decision to commute Stone's sentence days before he was due to report to prison marked the Republican president's most assertive intervention to protect an associate in a criminal case and his latest use of executive clemency to benefit an ally. Democrats condemned Trump's action, announced on Friday evening, as an assault on the rule of law.

Former special counsel Robert Mueller then sharply defended his investigation into ties between Russia and Donald Trump's 2016 presidential campaign, writing in a newspaper opinion piece Saturday that the probe was of "paramount importance" and asserting that a Trump ally, Roger Stone, "remains a convicted felon, and rightly so" despite the president's decision to commute his prison sentence.

The op-ed marked Mueller's first public statement on his investigation since his congressional appearance last July.

Mueller wrote that though he had intended for his team's work to speak for itself, he felt compelled to "respond both to broad claims that our investigation was illegitimate and our motives were improper, and to specific claims that Roger Stone was a victim of our office."

The op-ed chronicled the basis for the Stone prosecution, with Mueller recounting how Stone had not only tampered with a witness but also lied repeatedly about his efforts to gain inside information about Democratic emails that Russian intelligence operatives stole and provided to WikiLeaks, which published them in the run-up to the election.

The decision to commute the sentence of the 67-year-old Stone, who was convicted of lying to help the president and set to report to prison on Tuesday, was loudly celebrated by some in Trump's orbit as a triumph over deep state prosecutorial overreach.

Stone was among several Trump associates charged with crimes in former Special Counsel Robert Mueller's investigation that documented Russian interference to boost Trump's 2016 candidacy. The White House criticized Mueller's investigation and the prosecutors in Stone's case, saying the Left and its allies in the media attempted for years to undermine the Trump presidency.

(With input from agencies)