Fifteen-year-old Mattsson was trying to photograph a Police Cadet unit at the Armed Forces Day parade, when he was stopped by a police officer, who told him he couldn't photograph children without their parents' consent.
Quoting the law, Mattsson told her there is no restriction on photographing children in a public space.
Police then took over, at which point Mattsson began using his mobile phone to record the officer, who can be heard saying it is against the law to photograph army members and police officers under the Terrorism Act.
The law states that such protection is for officers working undercover or where there is reason to believe a photographer is collecting data for terrorist purposes.
As Mattsson continued to debate the legal rights of the officer's actions, his camera was taken off him and he claims he was pushed down some stairs before being detained.
Press photographers across the country have since voiced their outrage at the events and on his blog, Mattsson says he will consider taking legal action.
Director of Big Brother Watch Alex Deane told Journalism.co.uk it would support Mattsson in any future cases against the police.
"Some officers think that anti-terror legislation gives them blanket powers to hassle people whenever they want," he says. "This case is a very bad example of that. Confronted by bullying policemen, acting entirely outside their powers, this young man admirably stood up for himself and rightly maintained that he was entitled to take pictures in a public place. The police should apologise immediately and admit that they were in the wrong."
"If they don't, he should commence legal action and Big Brother Watch will help him all we can," he added.
But the Metropolitan Police Service say no complaints have yet been made against their officers.
Investigation
In a report by the British Journal of Photography, a spokesperson for the Metropolitan Police says they will be investigating the matter.
"It is clearly not the intention of the MPS to prevent people from taking photographs, although, as the public would expect, officers will remain vigilant, particularly in crowded public places," a comment released by the MPS says. "Any allegations or complaints about police treatment of photographers are taken very seriously by the MPS. Anyone who is unhappy with the actions of individual police officers can make a formal complaint, which will be thoroughly investigated."
"Although at this time we have not received a complaint about this incident and no allegations of crime have been made, we will investigate the circumstances. Our officers do receive guidance around the issue of photography through briefings and internal communications and we continue to drive this work forward."
Speaking to Journalism.co.uk today, a spokesperson confirmed they had still not had any complaints made in relation to the event.
The MPS statement follows news that that it apologised and paid damages to two journalists on Monday over a confrontation at last December's Greek Embassy protest.
In its July issue Amateur Photographer magazine will enclose a lens cloth with a printed letter to police and PSCOs, outlining street photography rights, according to a report by the BJP.
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