US President Donald Trump says the FBI's raid on the offices of his personal lawyer was a "disgrace" and an "attack on our country".
"I have this witch hunt constantly going on," he told White House reporters.
Officials in New York seized "privileged communications" between Michael Cohen and his clients, his lawyer said after the raid on Monday.
Documents regarding a payment to a porn actress were also seized, US media say.
Law enforcement acted on a "referral" from Special Counsel Robert Mueller, who is investigating suspected Russian meddling in the 2016 election.
Mr Trump criticised the investigation and described Mr Mueller's team as "the most biased group of people".
Mr Cohen has come under increasing public scrutiny since he admitted to making a $130,000 (€106,000, £92,000) payment to adult film star Stormy Daniels, whose real name is Stephanie Clifford, just days before the 2016 presidential election.
Ms Daniels claims she had a sexual encounter with Mr Trump shortly after his wife Melania gave birth to their son and says she was paid to keep quiet about the affair.
"Today the US Attorney's Office for the Southern District of New York executed a series of search warrants and seized the privileged communications between my client, Michael Cohen, and his clients," Mr Cohen's lawyer Stephen M Ryan said in a statement.
"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."
Mr Ryan called the raid "completely inappropriate and unnecessary".
"It resulted in the unnecessary seizure of protected attorney client communications between a lawyer and his clients. These government tactics are also wrong because Mr Cohen has co-operated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath," the statement said.
Mr Trump, who has denied having an affair with Ms Daniels, said he was unaware of Mr Cohen's payment to her and referred questions to the lawyer.