Officers refused to arrest campaigners celebrating the Houthi attacks in the Red Sea - but arrested a man who called Hamas terrorists.
As thousands of pro-Palestine demonstrators marched through London, one man reportedly shouted, "Yemen, Yemen, do us proud" thorough with a megaphone.
The crowd was heard chanting: "Turn another ship around" in response.
It comes after Niyak Ghorbani, 38 was arrested during a pro-Palestine demonstration on Saturday as he displayed a banner reading "Hamas is terrorist".
Officers refused to arrest campaigners celebrating the Houthi attacks in the Red Sea - but arrested a man who called Hamas terroristsGettyWhile grounded, Ghorbani shouted "shame on you", and said "I wrote down Hamas is a terrorist organisation – but they arrested me".
Officers say they intervened because of a suspected "breach of the peace" but have since "de-arrested" and subsequently released the Iranian IT professional.
"'I think the police are totally wrong," he told The Mail.
"It is a total double standard. It is one rule for the Palestinian protesters and another for me. It is hypocrisy."
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Critics have condemned the Metropolitan Police for their heavy-handed treatment of Ghorbani in comparison to a softer approach towards those "glorifying" Houthi terror attacks on British ships.
Tory MP Sir Iain Duncan Smith said: "I will be demanding a full inquiry with a public apology from the police. This Iranian man has done nothing wrong at all.
"For a man to be arrested and bundled to the floor by officers, who do not seem to know the difference between a peaceful organisation and a terrorist one, is astonishing."
On Saturday, an onlooker asked two officers: "Guys, you know what that reference is, the Yemen reference… the Houthis, proscribed terror organisation?"
Thousands of protesters have been gathering in London to take part in a pro-Palestinian demonstrations
ReutersOne of the officers shook his head while the other reportedly said: "I am listening to my earpiece, sorry."
Jewish Tory MP Andrew Percy said: "For months the police have ignored support for actual terrorist groups on our streets yet they find time to arrest someone pointing out a simple statement of fact."
A Met spokesperson said: "He was arrested after an altercation was ongoing and officers intervened to prevent a breach of the peace. He was arrested for assault.
"Officers then fully reviewed footage provided of the incident, and he was later de-arrested. The arrest was not made in relation to the placard."