Donald Trump has requested the US congress investigate claims former President Barack Obama ordered his telephones be wire-tapped during the election campaign.
The claims have been denied by Obama's spokesman.
The US President accused former Mr Obama of having Trump Tower telephones "wire tapped" during last year's election.
Trump did not offer any evidence or details, or say what prompted him to make the allegation.
Trump, whose administration has been under siege over campaign contacts with Russian officials, said in a series of early morning tweets that he "just found out that Obama had my 'wires tapped' in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
Obama spokesman Kevin Lewis said a "cardinal rule" of the Obama administration was that no White House official ever interfered in any Justice Department investigations, which are supposed to be conducted free of political influence.
"As part of that practice, neither President Obama nor any White House official ever ordered surveillance on any US citizen," Lewis said, adding that "any suggestion otherwise is simply false".
On Sunday, White House Press Secretary Sean Spicer said Mr Trump had requested "that as part of their investigation into Russian activity, the congressional intelligence committees exercise their oversight authority to determine whether executive branch investigative powers were abused in 2016".
"Reports concerning potentially politically motivated investigations immediately ahead of the 2016 election are very troubling," he said.
The White House did not immediately reply to inquiries about what prompted the president's initial tweets.
Trump compared the alleged activity to behaviour involving president Richard Nixon and the bugging of his political opponents.
"How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!" he tweeted, misspelling tap.
Trump said the wiretapping occurred in October. He ran the presidential transition largely out of Trump Tower in New York, where he also maintains a residence.
Trump's tweets came days after disclosures that Attorney General Jeff Sessions, during his Senate confirmation hearing, did not disclose his own campaign season contacts with Russia's ambassador to the United States.
Sessions, a US senator at the time, was Trump's earliest Senate supporter.
Trump's opening tweet on Saturday mentioned Sessions and claimed the first meeting Sessions had with the Russian diplomat was "set up by the Obama Administration under education program for 100 Ambs."
US intelligence agencies have concluded that Russia interfered in the campaign with the goal of helping elect Trump over Democrat Hillary Clinton, findings that Trump has dismissed.
The FBI has investigated Trump associates' ties to Russian officials. Congress is also investigating.
Trump has blamed Democrats for leaks of information about the investigation and the contacts.
It was unclear what prompted Trump's new charge. The president often tweets about reports he reads on blogs and conservative-leaning websites.
After tweeting Saturday about Obama, as well as about Arnold Schwarzenegger's decision to leave The New Celebrity Apprentice, Trump went to his golf club in nearby West Palm Beach.
Adam Schiff, the senior Democrat on the House Intelligence Committee, said in a statement that Trump was making "the most outlandish and destructive claims without providing a scintilla of evidence to support them".
Schiff added: "No matter how much we hope and pray that this president will grow into one who respects and understands the Constitution, separation of powers, role of a free press, responsibilities as the leader of the free world, or demonstrates even the most basic regard for the truth, we must now accept that President Trump will never become that man."