Owen Jones warned against blocking pro-Palestine protests on Remembrance Day.
The left-wing activist insisted demonstrators would turn out in “even greater numbers” if the rally planned for November 11 is banned.
Mr Jones said: “Just try and ban our 11th Nov protest against the mass slaughter of innocent people and what the UN warns is the risk of genocide.
“We’ll march in ever greater numbers if you do.
“As Harry Patch, the last surviving Tommy, once said: ‘War is organised murder and nothing else.'”
Mr Jones’s defiant tweet comes amid fears events commemorating the nation’s war dead could be disrupted by the march demanding a ceasefire in the Israel-Hamas conflict.
In a statement this afternoon, Rishi Sunak condemned the planned protest as “provocative and disrespectful”.
He said: “To plan protests on Armistice Day is provocative and disrespectful, and there is a clear and present risk that the Cenotaph and other war memorials could be desecrated, something that would be an affront to the British public and the values we stand for.
“The right to remember, in peace and dignity, those who have paid the ultimate sacrifice for those freedoms must be protected.
“I have asked the Home Secretary to support the Met Police in doing everything necessary to protect the sanctity of Armistice Day and Remembrance Sunday.”
Mr Sunak’s intervention comes after security minister Tom Tugendhat wrote to London Mayor Sadiq Khan, the Metropolitan Police and Westminster Council urging them to look at what powers they have to deal with the rally.
Scotland Yard has vowed to use all its powers to stop disruption of Remembrance events.
It said officers will be deployed across the capital that weekend as part of a “significant policing and security operation”.
The force said protest groups have not indicated plans to march on Remembrance Sunday, but a large demonstration is expected on the Saturday.
Organisers of the rally have pledged to avoid the Whitehall area where the Cenotaph war memorial – the focus of national remembrance events – is located.
