The protesters called on the British government and foreign minister David Miliband to intervene in Sayad Parwez Kambaksh's case ensure his release from jail.
Student journalist Kambaksh was imprisoned last month and faces possible execution. He was found guilty under Sharia law of insulting Islam for allegedly distributing articles from the internet on women's rights at Balkh University in northern Afghanistan.
Today's protest, in which 30 people gathered at Princes Gate, was organised by Faiborz Pooya of the Iranian Secular Society.
Pooya said: "Sayad Parwez Kambaksh is in prison and he is under great emotional pressure and he is in the same cells as Taliban prisoners, who have beaten him as they have been told by the guards that he blasphemed."
Earlier this week, Louise Arbour, the UN high commissioner for human rights, wrote to senior Afghan officials, including President Hamid Karzai, concerning the fate of Kambaksh.
David Miliband, the foreign secretary, and US Secretary of State Condoleezza Rice had already applied diplomatic pressure over the case, but Pooya said they have not been vocal enough.
Pooya said: "He has committed no crime and we are calling for his immediate and unconditional release. We are calling on the UK government and David Miliband to intervene. The UK has thousands of troops in the country and so should intervene immediately."
He added that the message was getting through to the Afghan government in Kabul and said that he too had been told that Kambaksh was likely not to be executed, but would remain in prison, although this has not been confirmed by any official channels.
"Commuting the death sentence is not enough. He should be released immediately. Unconditional freedom and nothing else. We will not stop this protest until that happens," he said.
The campaign to save Kambaksh has now attracted more than 75,000 signatures online at The Independent and almost 1000 people have joined the Facebook group.
The campaign to release Kambaksh is being supported by the International Federation of Journalists, the Committee to Protect Journalists and the Institute for War & Peace Reporting.
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