Donald Trump has claimed Barack Obama ordered for his offices in New York to be 'wire tapped'.
The allegation - made without showing any evidence - included calling Mr Obama a "bad, or sick, guy".
"Terrible!" the President said. "Just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
Soon after he added: "Is it legal for a sitting President to be "wire tapping" a race for president prior to an election? Turned down by court earlier. A new low!
"I'd bet a good lawyer could make a great case out of the fact that President Obama was tapping my phones in October, just prior to election!"
Mr Trump also suggested that Mr Obama was "sick".
The President said: "How low has President Obama gone to tapp [sic] my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!"
However, despite the severity of the claim, Mr Trump did not immediately provide evidence that Mr Obama was responsible for surveillance on his property.
Democratic commentators were quick to defend the former President.
Ben Rhodes, a foreign policy adviser to Mr Obama, tweeted: "No President can order a wiretap. Those restrictions were put in place to protect citizens from people like you."
And Representative Eric Swalwell, who sits on the House Intelligence Committee, told Fox News: "I think this is just the president up early doing his routine tweeting.
"Presidents don't wiretap anyone. These are pursued by the Department of Justice in accordance with the FBI and signed off by a judge."
Congressman Ted Lieu added: "Mr. President: If there was a wiretap at Trump Tower, that means a fed judge found probable cause of crime which means you are in deep s***."
The President has a long history of making unfounded and sometimes bizarre claims and it is not the first time he has used them to attack his predecessor.