The office of Donald Trump's lawyer has been searched, with FBI agents seizing several documents including details of a £92,000 payment made to porn actress Stormy Daniels.
A furious Trump said it was a "disgrace" that the FBI "broke into" his personal attorney Michael Cohen's office.
Mr Trump called the probe a "witch hunt," suggesting it was a distraction from serious issues, such as the consideration of a military response to Syria's apparent use of a chemical weapon on civilians over the weekend.
He also called special counsel Robert Mueller's Russia investigation "an attack on our country".
The raid was carried out by the US Attorney's office in Manhattan and based in part on a referral from Mueller, according to Mr Cohen's lawyer, Stephen Ryan.
In a statement Mr Ryan said: "The decision by the US Attorney's Office in New York to conduct their investigation using search warrants is completely inappropriate and unnecessary.
"It resulted in the unnecessary seizure of protected attorney client communications between a lawyer and his clients."
Commenting on the raid, Trump said: "I just heard that they broke into the office of one of my personal attorneys, a good man."
The law enforcement action will almost certainly amplify the public scrutiny on the payment to the actress, Stormy Daniels, which was made just days before the 2016 presidential election.
The fact the raid was referred to the Manhattan US Attorney's office suggests that the matter is not related to Russia.
Under Justice Department regulations, Mr Mueller is required to consult with Deputy Attorney General Rod Rosenstein when his investigators uncover new evidence that may fall outside his original mandate.
Mr Rosenstein then will determine whether to allow Mr Mueller to proceed or to assign the matter to another US lawyer or another part of the Justice Department.
Last week, Trump denied knowledge of the payment made by Mr Cohen and said he did not know where Mr Cohen had got the money. The White House has consistently said Mr Trump denies the affair.
Mr Cohen has admitted he paid Ms Daniels $130,000 out of his own pocket just days before the 2016 presidential election.
Ms Daniels has said she had sex with the president in 2006. She has been suing to invalidate the nondisclosure agreement she signed before the election and has offered to return the 130,000 dollars she was paid in order to "set the record straight".