US President Donald Trump has accused his predecessor of wire-tapping his phone a month before he was elected.
President Trump tweeted early on Saturday: "Terrible! Just found out that Obama had my "wires tapped" in Trump Tower just before the victory. Nothing found. This is McCarthyism!"
He went on to say that a court had earlier denied a wiretap request.
The US president has given no details to back up the claim - or suggested which court order he was referring to.
Mr Trump's tweets followed allegations made by conservative radio host Mark Levin, which were later picked up by Breitbart News, the website run by Steve Bannon before he became Mr Trump's chief strategist.
Mr Levin said there should be a congressional investigation into what he called President Barack Obama's "police state" tactics in his last months in office to undermine Mr Trump's campaign.
Breitbart summarises Mr Levin's accusations, which say that "the Obama administration sought, and eventually obtained, authorisation to eavesdrop on the Trump campaign; continued monitoring the Trump team even when no evidence of wrongdoing was found; then relaxed the NSA (National Security Agency) rules to allow evidence to be shared widely within the government".
Media reports in the past few weeks have suggested the FBI had sought a warrant from the foreign intelligence surveillance court (Fisa) last summer in order to monitor members of the Trump team suspected of irregular contacts with Russian officials.
The warrant was first turned down but then approved in October, according to the media reports. There has been no official confirmation and it is also not clear if this evolved into a full investigation.
There has been no comment yet from Mr Obama.
But Ben Rhodes, who was Mr Obama's foreign policy adviser and speechwriter, addressed Mr Trump's claims in a tweet, saying: "No President can order a wiretap. Those restrictions were put in place to protect citizens from people like you."
Other Democrats said that if a wiretap had taken place it would have been because a judge had found probable cause that a crime was being committed.
Partisan maelstrom: Analysis by Anthony Zurcher, BBC News, Washington
As the Trump administration continues to be bedevilled by a drip-drip of revelations about ties to Russian officials during and after last year's election, the president has seemingly settled on the identity of the malevolent figure behind the turmoil.
It was the former president, Mr Trump asserts, who is guilty of meddling in the 2016 campaign, not Russia. Mr Obama, he says, is the one whose deeds merit investigation.
The president's early morning tweets follow an interview on Tuesday in which he accused Mr Obama and "his people" of orchestrating recent political protests across the US and of being behind the government leaks that have embarrassed the White House.
There is scant evidence supporting these allegations, but charges like these fit a pattern. Mr Trump is at his sharpest when pushing back against perceived antagonists, such as Republican primary opponents like Jeb Bush, establishment conservatives who resisted his nomination or Hillary Clinton in the election.
Now Mr Trump is returning to his favourite political foil - a necessity given the current Democratic power vacuum in Washington. It could mark the beginning of a massive water-muddying effort in which any forthcoming investigatory revelations are swept up in a growing partisan maelstrom.
Mr Trump had tweeted: "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."
He called the alleged tapping "a new low" and said "This is Nixon/Watergate" - referring to the most notorious political scandal of 1972, which led to the downfall of President Richard Nixon after a web of political spying, sabotage and bribery was exposed by the media.
He also called it McCarthyism - the persecution for US Communists and their allies led by Senator Joe McCarthy in the 1950s.
One of the first to react to Mr Trump's claims was Democrat Eric Swalwell, who sits on the House Select Committee on Intelligence. His Twitter feed simply carried an image of one of the president's tweets and said: "You're not credible."
But Alexey Pushkov, the head of the Foreign Affairs Committee in Russia's State Duma (lower house of parliament), accused Mr Obama of being a "wiretapping maniac", listening in to "Merkel, Hollande and half of Europe".
Republican Senator Lindsey Graham told a town hall meeting at Clemson University that he was "very worried that our president is suggesting that our former president has done something illegally".
He said that if Mr Obama had acted illegally, or had even obtained a warrant lawfully to monitor the Trump campaign, "it would be the biggest political scandal since Watergate".
Mr Trump has been reeling from accusations of links between his campaign team and Russia, following an intelligence report that Moscow was involved in hacking in order to get Mr Trump elected.
In the latest twist, his Attorney-General Jeff Sessions has been forced to remove himself from an investigation into the Russian role.
This followed revelations that he had met the Russian ambassador during the campaign, despite denying this at his confirmation hearings.
Mr Trump's National Security Adviser Michael Flynn was forced to resign after four weeks in office for misleading the White House over his contacts with the Russian envoy during the election campaign. Sanctions against Russia were allegedly discussed.