The man, who refused to give any name other than Mr Broccoli, warned the planet is in “incredibly hot water” because of climate change as he appeared on Good Morning Britain.
But he was met with opposition from Morgan, who claimed: “You don’t know the science.”
The guest, who appeared on the show in green face paint and broccoli floret-style headwear to promote a plant-based diet, replied: “I’m just a broccoli at the end of the day.”
He then produced a banana from his pocket and held it to his ear while pretending to be on the phone.
Extinction Rebellion protesters shut down London roads
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The activist is part of the Animal Rebellion movement, an offshoot of climate group Extinction Rebellion, and was arrested on Sunday as part of the environmental protests in London.
Referencing climate protests, he said: “I want people to feel like they can come and take part in this protest.”
During the interview, Reid asked: “Can I just ask a straightforward question and without any comedy – are you a plant?
A man dressed as a broccoli is arrested during an Extinction Rebellion protest in Oxford Street, London, 14 October 2019.
(Robert Greener/PA Wire)
“Because your argument dressed as a broccoli just strikes me as so absurd that you’re wearing a costume but you’re trying to get a serious point across about veganism.
“Are you actually not part of Animal Rebellion and Extinction Rebellion at all but somebody who is trying to send up the whole movement?”
When Reid asked if his appearance was a “joke”, he said: “I think given the gravity of the situation, some levity and humour is important.”
Viewers reacted to the interview using the hashtag #MrBroccoli, with some more keen on the vegetable-themed protester than others.
Twitter user Glen O’Hara, who was retweeted by Reid, said the interview was “absolutely peak 2019”.
While Elliott Wright of The Only Way Is Essex said Mr Broccoli was a “moron”, and that Morgan should have “served him up on a plate”.
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Many social media users agreed that watching the segment was a strange way to wake up in the morning, with one asking: “What in the hell is going on here?”
Asked about his arrest, a spokeswoman for the Metropolitan Police said they could not comment on individual cases.
More than 1,300 people have already been arrested during the protests so far, which are part of an “international rebellion” taking place in cities around the world.
Additional reporting by Press Association.
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