Donald Trump has described an FBI raid of his lawyer's office as "an attack on our country".
Federal officers visited Michael Cohen's office and home on Monday night and took "communications" amid controversy over payments the lawyer made to adult film actress Stormy Daniels.
Mr Cohen has been at the centre of allegations made by Ms Daniels that she was paid to keep quiet after having sex with Mr Trump.
The President called the raids a "disgrace" and "witch hunt", adding: "When I saw this, when I heard about it, that is a whole new level of unfairness."
Stephen Ryan, who is Mr Cohen's lawyer, said certain "communications" had been seized.
He added: "I have been advised by federal prosecutors that the New York action is, in part, a referral by the Office of Special Counsel, Robert Mueller.
"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."
A spokesman for Special Counsel Mueller was not able to comment.
As he prepared to meet with the defence secretary and join chiefs of staff on Monday evening, Mr Trump said: "I just heard that they broke into the office of one of my personal attorneys, a good man.
"I've been saying it for a long time I wanted to keep it down. We've given I believe over a million pages worth of documents to the special counsel.
"They continue to just go forward and here we are talking about Syria. We're talking about a lot of serious things with the greatest fighting force ever.
"And I have this witch hunt constantly going on for over 12 months now and actually much more than that. You could say it was right after I won the nomination instead."
He called the investigation "ridiculous" and said it was run by the "most biased group of people", adding that "no collusion" had been found.
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Mr Mueller is investigating claims Russia meddled in the 2016 US presidential election and possible collusion between Russia and the Trump campaign.
Mr Trump added: "Why don't I just fire Mueller? Well I think it's a disgrace what's going on. We'll see what happens."
According to Mr Ryan, Mr Cohen has been co-operating with the authorities and has already handed over thousands of documents to investigators.
The New York Times reported a source as claiming emails, tax documents and business records were seized on Monday.
Last month, Ms Daniels, real name Stephanie Clifford, announced she was suing Mr Trump after she was paid to keep quiet about their 2006 affair.
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She claims Mr Trump did not sign the Hush Agreement so that he could distance himself from it, and from Ms Daniels.
Mr Cohen said the payment was made by him and he was never reimbursed by the Trump campaign.
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