Donald Trump has accused Barack Obama of tapping his telephones at Trump Tower in October, before he was elected US president.
Mr Trump unleashed a flurry of tweets at about 6.30am Saturday morning accusing the former president of using wire taps to spy on him at his New York base.
“Terrible! Just found out that Obama had my ‘wires tapped’ in Trump Tower just before the victory. Nothing found. This is McCarthyism!,” he tweeted.
He president compared the alleged surveillance to Watergate, the 1970s scandal that brought down president Richard Nixon: “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!”
Mr Trump also appeared to make a threat of legal action, saying “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 the Election!”.
Mr Trump did not provide any additional evidence to back up his claims and it was unclear what information he was relying on.
Barack Obama has not yet responded to the accusations.
The Trump administration has come under increasing pressure this week over its connections to Russian officials, after it emerged his attorney general Jeff Sessions had twice talked with Russia’s ambassador to Washington during the campaign.
Mr Sessions recused himself from an investigation into Russian interference in the US election.
Mr Trump has continued to defend Mr Sessions, labelling the accusations a “witch hunt” and saying the attorney general could have been more accurate in what he said about his contacts with Russian officials but blaming Democrats for blowing up the controversy for political reasons.
“Jeff Sessions is an honest man. He did not say anything wrong. He could have stated his response more accurately, but it was clearly not intentional,” Mr Trump said in a statement on Thursday.
“The Democrats are overplaying their hand,” Trump said in the statement. “It is a total witch hunt!”