A protester holding a sign accusing Home Secretary Priti Patel of being a 'fascist' was 'bundled' into a police van outside the Conservative Party conference, he claims.
Liam Geary Baulch was holding a cardboard sign with the message 'Priti Fascist' written on it.
He says he had it confiscated and was then arrested for a breach of the peace, after asking for it back.
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Video footage shows him being led into a police van.
He was later 'de-arrested'.
GMP said a 28-year-old man was arrested to 'prevent further breach of the peace' after 'behaving aggressively' towards an officer.
The protests came as the Home Secretary gave a speech at the party conference, in which she said the courts would be given more powers to stop people intent on 'causing disruption' from attending protests.
Liam said: "Within 30 seconds of holding this sign outside the Conservative party conference it was confiscated by Manchester police for 'offensive language'.
"When I asked to have my property back, I was arrested for breach of the peace.
"As Priti Patel laid out her crackdown on protest inside, I was in the back of a van for simply standing alone with a piece of cardboard.
"If anything I'm more convinced now that this sign is the truth."
The arrest was criticised by NetPol, The Network for Police Monitoring, a group which monitors public order, street and protest policing.
NetPol wrote in a tweet: "A protester standing outside Conservative Party conference in Manchester with a handmade sign reading 'Priti Fascist', during the Home Secretary's speech to delegates, was arrested and bundled into a police van.
"He's not wrong, is he?
"He was subsequently "dearrested" but how on earth does @gmpolice justify this fundamental denial of rights to freedom of expression and assembly?"
A GMP spokesperson said: "A man was behaving aggressively towards an officer at around 11:30am this morning (Tuesday 5 October) on Peter Street, when another officer intervened and requested he step back multiple times.
"A 28-year-old man was arrested to prevent further breach of the peace, and was given the option of being taken into custody, or leaving the area peacefully.
"The man chose the latter, and was de-arrested a short time later."
