The FBI raided Donald Trump's Mar-a-Lago estate in Florida and broke into his safe in a stunning escalation of the legal scrutiny faced by the former US President.
Agents executed a search warrant, authorised by a judge, which appeared to be to do with boxes of documents containing classified information that Mr Trump brought with him to Florida when he left the White House.
There was outrage among Republicans, who accused Democrats of "weaponising" the justice system.
The raid took place on Monday and was made public by Mr Trump himself in a lengthy statement.
Mr Trump, who was not at Mar-a-Lago at the time, said: "These are dark times for our Nation, as my beautiful home, Mar-a-Lago in Palm Beach, Florida, is currently under siege, raided, and occupied by a large group of FBI agents.
"After working and cooperating with the relevant Government agencies, this unannounced raid on my home was not necessary or appropriate."
He added: "Such an assault could only take place in broken, Third-World Countries. They even broke into my safe!"
It was believed Joe Biden and White House officials did not know about the raid in advance, and they were said to have been blindsided.
The FBI and the US Justice Department declined to comment.
The Justice Department has for months been investigating Mr Trump's removal of White House records to Florida after he left office.
Earlier this year, the US National Archives notified Congress that it had recovered about 15 boxes of White House documents from Mar-a-Lago, some of which contained classified materials.
That reportedly included classified texts and correspondence from Mr Trump's predecessor Barack Obama.
The National Archives referred the matter to the Justice Department, leading to the investigation and the subsequent search warrant for Mar-a-Lago.
Agents reportedly searched parts of the resort used by Mr Trump as his office and personal living quarters.
Under US law the Presidential Records Act, introduced after Watergate in the 1970s, prohibits the removal of classified White House documents to unauthorised locations. Mishandling classified information is potentially punishable with a prison term.
In his statement Mr Trump said the decision to have FBI agents search his home was "prosecutorial misconduct, the weaponisation of the Justice System, and an attack by Radical Left Democrats who desperately don't want me to run for President in 2024."
He added: "What is the difference between this and Watergate, where operatives broke into the Democrat National Committee? Here, in reverse, Democrats broke into the home of the 45th President of the United States."
The Justice Department did not say whether US Attorney General Merrick Garland had personally authorised the search.
Stephen Miller, who was a senior adviser to Mr Trump in the White House, claimed the FBI had become a "Praetorian Guard".
He told Fox News: "This is an abomination. You have the sitting president Joe Biden, through his Justice Department, his FBI, conducting a raid on the person who is presumed to be his opponent in the next election."
Eric Trump, the former president's son, said: "They will do anything they can to take him out.
"I hope he goes out and beats these guys again. This is Venezuela. This is banana republic antics."
Marco Rubio, the Republican senator, said: "Using government power to persecute political opponents is something we have seen many times from third world Marxist dictatorships. But never before in America."
Kristi Noem, the Republican governor of South Dakota, added: "Using the criminal justice system in this manner is un-American."
Some supporters of Mr Trump said he was being "martyred" and it could help him secure the Republican presidential nomination in 2024.
But others said it would be highly damaging for his chances and meant Republicans would turn to other candidates, including former vice president Mike Pence and Florida governor Ron DeSantis.
One Republican donor, who recently met with Mr Trump at Mar-a-Lago, told the Telegraph it would "rip the 2024 Republican primary wide open."
He said: "The Trump era is over and it's anybody's ball. I think the biggest winner here is Mike Pence."
He added: "This is a terrible precedent. Every president moving forward will be prosecuted for something."
David Axelrod, former chief strategist for Barack Obama, said it would make Mr Trump more likely to run for the White House again.
He said: "This has never been done before in the history of this country. The idea they would raid the home of a former president, it's the biggest stakes you can have.
"Merrick Garland is a notoriously cautious person. He would not have authorised it unless they believe they had significant evidence a crime had been committed."