Seven Royal Marines were arrested on suspicion of murdering a Taliban fighter after a video clip was discovered on a laptop, it has emerged.
The Marines, all believed to be members of Bickleigh-based 42 Commando, were detained on Thursday afternoon.
Meanwhile a Facebook page supporting "theroyalmarine7" which was set up within a few hours, last night had amassed nearly 18,000 "likes".
A spokesman for the Ministry of Defence said it would not be appropriate to comment in detail. However, he added that if proved, the servicemen would face the full weight of the law.
"Royal Marines are a highly effective fighting force," he said."We take these incidents and these allegations extremely seriously.
"They will be robustly investigated and the servicemen will be dealt with very seriously if they are proved."
The seven Royal Marines arrested were serving as part of the Plymouth-headquartered 3 Commando Brigade, which was in charge of the six-month summer tour of duty in Helmand province last year.
It is understood that the incident under scrutiny followed a firefight with a member of the insurgency.
However, it only came to light following an arrest last week on a separate matter, by civilian police, of a man who had served in Afghanistan.
The man's laptop was subsequently examined and police stumbled across a video clip which is understood to have shown the Royal Marines in a compound in Helmand with what appears to be an injured Taliban fighter.
It is further understood that the conversation in the background concerns what to do with the wounded man and whether to administer first aid. The video footage ends without the insurgent's fate being revealed.
The investigation is being led by the Royal Military police, a branch of the army.
The seven Royal Marines have been detained in custody, but the MoD has not revealed where.
The investigating officer will make a report to the Director of Service Prosecutions, an independent QC who is accountable to the Attorney General rather than to the armed services. If there is enough evidence to press ahead, the men will be subject to a court martial, which has the same sentencing powers as a Crown Court.
Social networking sites and unofficial websites for serving and former military personal have been awash with comments on the case, many focusing on the Rules of Engagement, the laws established by the Geneva Convention under which the armed forces operate.
Scores of contributors on the Facebook page set up in support of the Marines angrily note the incident at the centre of the controversy as one in which no civilians were injured, just one insurgent who had opened fire on the marines.
During the six-month tour, seven members of 42 Cdo were killed and dozens seriously injured.