British activist Tommy Robinson vowed to burn an image of Iran's Supreme Leader as anti-government activists plan to perform the protest on Friday in solidarity with a man found dead in Iran after filming himself doing so.

Robinson appeared on the YouTube channel Tousi TV on Tuesday, where host Iranian-American host Mahyar Tousi relayed a request from an audience member for the outspoken activist to take part in the symbolic act against Ali Khamenei.

"Print a big picture of the scumbag, and I will happily burn it," said Robinson, 42, an anti-migration activist whose legal name is Stephen Yaxley-Lennon.

Iranians on social media have launched a campaign to burn Khamenei's image after Omid Sarlak was found dead in a car in the city of Aligoudarz in western Iran on Sunday.

His death came shortly after posting a video of himself burning a picture of the 86-year-old theocrat with a speech by Iran's last shah playing in the background.

Local police described it as a suicide, but family members and rights activists blamed authorities. The United States on Wednesday called the death "suspicious" and suggested Tehran was responsible.

His death came as senior Iranian clerics renewed calls for severe punishment of those who insult or threaten the Supreme Leader, which is a crime in the Islamic theocracy.

One prominent official said such acts amount to "waging war against God" and warrant the death penalty.

Robinson was cleared by a UK court on Tuesday on charges relating to his refusal to provide the PIN for his mobile phone to investigators, which is an offence under counterterrorism laws.

He thanked Elon Musk, the world's richest man and in recent years a strong advocate of right-wing commentators, saying the owner of the X social media platform paid his legal fees.