His comment on Twitter came after former FBI chief Robert Mueller was appointed special counsel with sweeping powers to head the probe.
"This is the single greatest witch hunt of a politician in American history!" Trump wrote.
Also via Twitter, Trump accused former president Barack Obama and his 2016 rival, Democrat Hillary Clinton, of unspecified "illegal acts."
This is the single greatest witch hunt of a politician in American history!— Donald J. Trump (@realDonaldTrump) May 18, 2017
"With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special councel [sic] appointed!" he wrote.
Trump has denied any links to Moscow, but the appointment of a special counsel dramatically raises the stakes in a crisis threatening to paralyze his presidency.
The Republican leader, who has struggled to shake off suspicions that Russia helped put him in the White House, has been accused of seeking to block the investigation by firing FBI chief James Comey.
Republicans have largely stood behind Trump in the first months of his presidency as the FBI and congressional investigations into Russia's election meddling intensified. But GOP lawmakers have grown increasingly anxious since Trump fired Comey, who had been leading the bureau's probe — and after Comey associates said he had notes from a meeting in which Trump asked him to shut down the investigation into the Russia ties of former National Security Adviser Michael Flynn.
Trump is preparing to leave town Friday on his first foreign trip, and aides have been hopeful the journey will be a chance for the administration to get back on track after weeks of chaos and distractions.
Sen. Lindsey Graham, R-S.C., speculated Trump was probably happy to get out of town — "and a lot of us are glad he's leaving for a few days."
His advice to the president: "Stay disciplined, stay focused and deliver on the world stage."
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