Britain First leader Paul Golding has warned MPs and journalists will meet their "miserable ends at the hands of the movement".
In a video released a week after he was released from prison, the far-right leader threatens to "confront and oppose every traitor in this country."
Mr Golding was jailed for eight weeks after breaching a High Court ban and entering a mosque.
To a backdrop of images of Prime Minister Theresa May, Labour leader Jeremy Corbyn and former Prime Minister Tony Blair, Mr Golding says: "All the politicians, all the journalists who have spent their careers undermining our nation, you will succumb to this movement eventually."
He adds: "I can promise you, from the very depths of my being, you will all meet your miserable ends at the hands of the Britain First movement. Every last one of you."
Mr Golding also makes the claim he was put in prison by a "corrupt left-wing establishment" in the hopes it would "moderate" his behaviour.
The anti-fascism charity Hope not Hate said Mr Golding's words must be taken very seriously.
Mr Golding was given an eight-week jail sentence after an injunction was requested by Bedfordshire Police following a series of “mosque invasions” in which the far-right group filmed themselves going into mosques to confront imams or worshippers.
Last year, Britain First's co-leader Jayda Fransen was found guilty of religiously aggravated harassment after shouting abuse at a Muslim woman wearing a hijab.
She was fined nearly £2,000 for wearing a political uniform and shouting at the woman during one of the group's "Christian patrols".
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