David Crouch of the Media Workers Against the War campaign group told Journalism.co.uk that around 100 people had attended the demonstration and that Zaidi had shown 'bravery to expose the media situation'.
The Iraqi journalist had 'used the opportunity' of an international press conference, he said, to highlight concerns with the Bush administration's foreign policy.
Zaidi is thought to have been beaten after the event, according to the investigating judge in the case, as reported by the Guardian, and remains in custody.
Crouch, speaking to Journalism.co.uk after the demonstration, said that he was pleased with the turnout, which included journalists, and the 'huge press interest'.
A 'huge stack' of shoes were collected, he said.
The letter signed by journalists and politicians said that Zaidi is 'guilty of nothing but expressing Iraqis' legitimate and overwhelming opposition to the US-led occupation of their country.'
The letter asks that US ambassador, Robert Holmes Tuttle, helps 'guarantee his [Zaidi's] safe treatment' and negotiate his 'immediate release from custody'.
The journalists who have signed the letter include Nick Davies, Eamonn McCann, staff from the UK-based Al-Quds al-Arabi newspaper, and around 40 members of the Media Workers Against the War organisation.
Politicians in support include Tony Benn and Jeremy Corbyn MP.
Update: AFP reported that around 40 people attended the demonstration, according to Scotland Yard.
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