Hundreds of forces' veterans have gathered outside Parliament calling for an immediate retrial for one of their former comrades who was convicted of murdering an Afghan insurgent.
Royal Marine Sergeant Alexander Blackman, 41, received a life sentence two years ago for killing the wounded captive in Helmand province in September 2011.
He unsuccessfully challenged the outcome in the Court Martial Appeal Court, however his minimum sentence was reduced to eight years.
Supporters say Blackman believed the insurgent was dead already and has been the victim of a gross miscarriage of justice.
Addressing the rally today, co-organiser Jeff Little, who reached the rank of corporal, called on the Government to instigate an immediate retrial.
He said: "We feel that this Government has let down one of our own after serving our country proudly, like we all have."
Before the protest, a Ministry of Defence spokeswoman said regulations prohibited members of the military from attending political protests, marches, rallies or demonstrations.
"Any gathering which seeks to protest against a decision taken by the legal system or the Government falls into this category," she said.
Serving officers had still expressed their intention to attend.
Mr Little insisted the gathering was not political in nature.
"This is for one reason and one reason only - in support Alex Blackman," he said.
"We're here to show him our support and to show those people in there we support him."
Footage from another marine's helmet-mounted camera showed Blackman quoting Shakespeare before shooting the insurgent at close range with a 9mm pistol.
He told the victim: "There you are. Shuffle off this mortal coil, you c***. It's nothing you wouldn't do to us."
Mr Little said Blackman had been under immense pressure, having led a team of 15 men for five months in an extremely hostile environment.
He said: "They were aware that hundreds of their comrades had already been killed or maimed by IEDs (improvised explosive devices). The psychological impact was devastating.
"Fire-fights with the Taliban was common almost every day so too were deaths and life-threatening injuries."
Mr Little said Blackman admitted what he did was wrong, however an argument could be made that the Geneva Convention did not cover Taliban fighters because they should be classified as terrorists.