Jeremy Corbyn has launched an attack on the right-wing press and warned "change is coming" after a series of stories about his Cold War past.
The Labour leader earlier flatly denied that he was a spy for communist Czechoslovakia during the Cold War.
He said claims, which originally appeared in The Sun, that he passed information to an agent of the Czech StB intelligence agency during the 1980s as "nonsense".
Mr Corbyn has now released a video on social media attacking Conservative supporting newspapers for publishing "lies and smears".
The stories show "just how worried the media bosses are" about the pospect of a Labour government, he claimed.
In the video, Mr Corbyn says: "In the last few days, The Sun, The Mail, The Telegraph and The Express have all gone a little bit James Bond.
"They've found a former Czechoslovakian spy whose claims are increasingly wild and entirely false.
"He seems to believe I kept him informed about what Margaret Thatcher had for breakfast and says he was responsible for either Live Aid or the Mandela Concert, or maybe both.
"It's easy to laugh, but something more serious is happening. Publishing these ridiculous smears that have been refuted by Czech officials shows just how worried the media bosses are by the prospect of a Labour government.
"They're right to be. Labour will stand up to the powerful and corrupt - and take the side of the many, not the few.
"A free press is essential for democracy and we don't want to close it down, we want to open it up. At the moment, much of our press isn't very free at all.
"In fact it's controlled by billionaire tax exiles, who are determined to dodge paying their fair share for our vital public services.
"The general election showed the media barons are losing their influence and social media means their bad old habits are becoming less and less relevant.
"But instead of learning these lessons they're continuing to resort to lies and smears. Their readers, you, all of us, deserve so much better.
"Well, we've got news for them: change is coming."
Mr Corbyn was challenged about the allegations by a journalist from the Daily Mail during a question and answer session at the EEF manufacturers' conference in London, where he had delivered a keynote speech.
He replied: "I am sorry the Daily Mail has reduced itself to reproducing some nonsense that was written in The Sun."
Asked directly by BBC Breakfast presenter Steph McGovern, who was acting as compere for the event, if he was a "Czech spy", he said: "No."
His denial came after he threatened Tory MP Ben Bradley with legal action over a tweet claiming he "sold British secrets to Communist spies".
According to the original Sun report, documents unearthed in the StB archives showed that Mr Corbyn met a Czech agent on at least three occasions, including twice in the House of Commons, during the 1980s and was given the codename Cob.
The Labour leader's office acknowledged that he had had tea in the Commons with a Czech diplomat, but said any claim he was "an agent, asset or informer for any intelligence agency is entirely false and a ridiculous smear".