Cage director Muhammad Rabbani was found guilty of obstruction after refusing to submit passcodes to his electronic devices to the police.
Rabbani said he could not provide access to his electronics because they contained information about alleged acts of torture carried out by the United States in Guantanamo Bay.
He explained that he was transporting the highly sensitive and confidential legal material on those devices. "There were around 30,000 [documents] which I was especially uncomfortable handling and I felt an enormous responsibility to try and discharge the trust that was given to me," he said.
Following a one-day trial, Rabbani was – under Schedule 7 of the Terrorism Act 2000 – convicted on a terrorism offense charge. The act affords police the authority to detain, question or determine "commission, preparation or instigation of acts of terrorism".
Law enforcement has stopped more than 17,501 people since June.
The 36-year-old said he has been stopped almost a dozen times, seven of which were Schedule 7 related.
During the interrogation of three officers involved with Rabbani's detainment, a senior officer admitted that defender had been specifically profiled.
"I considered that although the police were in law entitled to ask questions so that they could satisfy themselves I was not engaged in terrorist activity, that did not justify in addition being required to expose all the sensitive contents of my phone to being copied and undoubtedly disseminated not just to police but to intelligence services and possibly elsewhere in the world – an unjustifiable, uncontrolled acquisition of material," he detailed.
Senior District Judge Emma Arbuthnot said Rabbani's decision to not surrender the requisite passcodes to the devices was a "calculated risk." Upon consideration of the intended purpose of the documents, the judge handed down a one-year suspended sentence and £600 fine.
But, a Terrorism Act offense conviction will blemish his previously clean criminal record.
The activist said he will appeal the verdict because he was wrongly convicted by a flawed stop and search law. "If privacy and confidentiality are crimes, then the law stands condemned," he said, addressing a group of supporters outside the court.
Head of a counter-terrorism command at Scotland Yard, Commander Dean Haydon, said the verdict was important. "We are committed to ensuring the power is used appropriately and proportionately, as it was in this case."
He added that the Met police was still in possession of Rabbani's devices, and would be "continuing its efforts to examine the contents".
The Cage group have been raising the profile of inhumanity affecting the Guantanamo detainees since 2003.
Rabbani reportedly told The Intercept he believed the authorities wanted to copy the information, saying he had been searched on several prior occasions but had never been challenged by the relentless pursuit of the police to access the information.