Jeremy Corbyn has warned press barons “change is coming” as he accused them of printing “lies and smears” over his contacts with a Czech spy in the 1980s.
The Labour leader said newspapers had gone a “little bit James Bond” in their coverage of his meetings with diplomat and agent Jan Sarkocy.
He suggested the reporting showed how “worried” media bosses were by the prospect of a Labour government.
He earlier faced questions about the story at an event in London.
Details of a file held on Mr Corbyn by the StB, the Communist-era Czechoslovakian intelligence agency, were first reported by The Sun almost a week ago.
It reported contacts Mr Corbyn is alleged to have had with Mr Sarkocy, who was working in the Czechoslovakian embassy in London, towards the end of the Cold War in 1986 and 1987.
Czech officials have since told the BBC Mr Corbyn was a “person of interest” to their intelligence agencies but not an informant, contradicting claims made by Mr Sarkocy.
The story has been widely reported by British newspapers, prompting calls by Theresa May for Mr Corbyn to be open about his past actions and one cabinet minister to accuse him of “betraying” Britain.
Mr Corbyn has previously declined to comments on the claims but earlier on Tuesday at an event in London he was asked directly whether he was a Czech spy, replying “no”.
The Labour leader has now upped the ante, releasing a video message to supporters accusing newspapers of reporting “increasingly wild and entirely false” claims made by Mr Sarkocy.
“In the last few days, The Sun, The Mail, The Telegraph and The Express have all gone a little bit James Bond.
“It’s easy to laugh, but something more serious is happening,” he said.
“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.”
Mr Corbyn said the right-wing press had become less powerful in the era of social media and “their bad habits were becoming less and less relevant”.
“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.
“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.”
The Czech archive’s director said Mr Corbyn, who was a newly elected MP at the time of the meetings with Mr Sarkocy, was seen as a possible “contact” by the Czechoslovakian authorities because of his political views but no more than that.
Svetlana Ptacnikova said there was no evidence Mr Corbyn had been a “secret collaborator”.