His comment came after former FBI chief Robert Mueller was appointed special counsel with sweeping powers to head the probe.
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."
"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.
Mueller's appointment has come as Trump fends off reports he shared US secrets with Russian officials in the Oval Office, and had asked Comey to drop his investigation into former Trump national security advisor Michael Flynn.