President Trump on Thursday dismissed the ongoing investigation into his alleged ties to Russia as a "witch hunt," one day after the Justice Department named former FBI Director Robert Mueller as the special counsel in charge of the investigation.

"This is the single greatest witch hunt of a politician in American history," Trump tweeted Thursday morning.

Trump also complained that no similar steps were ever taken to police the last two Democratic administrations.

"With all of the illegal acts that took place in the Clinton campaign and Obama administration, there was never a special councel [sic] appointed!" Trump wrote.

The decision by Deputy Attorney General Rod Rosenstein to appoint special counsel was widely praised by Democrats and Republicans, although some GOP lawmakers said they haven't seen enough evidence yet to warrant that step.

The news from the Justice Department came a week after Trump fired FBI Director James Comey. The FBI was looking into ties Trump's associates had to Russia, and Democrats have charged that Trump fired Comey to hinder the investigation.