By Hadeel Al Shalchi and Erika Solomon

ALEPPO, Syria (Reuters) -- Rebel fighter Ahmed chuckles as he tells the story of the Syrian army sniper who was captured and executed by rebels, shot dead after a "trial" conducted at his own graveside.

He says Rami the sniper, firing from a high minaret at the local mosque, killed many rebels and civilians before he was seized by fighters from Ahmed's Amr bin al-Aas brigade which operates around the northern Syrian town of Azaz.

"We took him right to his grave and, after hearing the witnesses' statements, we shot him dead," said Ahmed, who has a rich, black beard and a broad, easy smile.

The rebels are waging a 17-month uprising against President Bashar Assad, during which government forces have frequently been accused of carrying out summary executions and other violations against fighters and civilians.

But video footage of a purported execution of pro-Assad militia and rebels making a gloating victory tour of an Aleppo police station still strewn with bodies, suggest both sides are capable of brutalities. And rebels themselves say they have no cause to show mercy to their opponents.

One of the witnesses at Rami's "trial" was a man who said he lost two sons to the sniper. But beyond a few witness statements, the justice dispensed appeared to be summary, at best.

Ahmed said his forces often capture a handful of soldiers in battle, and would know the names and faces of certain commanders or soldiers who were loyal to Assad. They would create courts for the captured men, and execute them.

"The culture of lawyers has long passed. I mean a guy like that," Ahmed said, referring to Rami the sniper. "What do you think his fate should be?"

Sharia courts

Further south in Aleppo, a dozen prisoners stand against a wall with their hands above their heads, hours after surrendering to rebels who overran their police station on the southern fringes of the city, not the same building as the one shown in the video.

The former police officials face an uncertain fate at the hands of fighters who said they would be put on trial in front of a Sharia (religious) court. Sharia justice, implemented in its most literal form, can involve eye-for-an-eye retribution, up to and including the death penalty.

Others who chose to resist the rebel attack on the police station in Salheen district were overwhelmed and killed after a battle which activists said lasted several hours.

One of the policemen said he received a call from the rebels on Monday night, a few hours before the attack started, saying that if he wished to surrender he should bring his men out with their hands raised, chanting "God is Greater".

"I told my guys: 'Tomorrow when they come in, drop your weapons'. This morning we didn't fire a single bullet," he said, in a school building being used by the Aleppo rebels as a base and a prison for the captives.

"Of course at that moment we were very frightened, but they kept saying we would be safe so we went out and everything went as planned," he said.

None of the captives, wearing regular civilian trousers and shirts, showed signs of having been mistreated since they gave themselves up.

"Notice we haven't hit them in the face," said Abu Bakr, one of the rebel leaders at the school base. "Have we hit you once?" he asked, and was answered by a vigorous shaking of heads by his captives.

"These prisoners will be transferred to sharia courts to be tried by judges and clerics who have defected from the regime, and religious scholars," he said, adding their fate was in the hands of the rebel 'courts'.

"It depends on what the court decides and whether they have blood on their hands. Whatever they decide we will commit to that. For now they are going to prison."

As he spoke, a cleric in white turban and cloak entered and began talking to the captives.

Despite the reassurances, signs of growing brutality are present across Aleppo, Syria's biggest city and a possibly decisive prize in the 17-month struggle for power between Assad and the rebel forces.

Next to a nearby building two bodies lay on the ground -- a police commander and his driver, according to the rebels -- the last remaining corpses of the men killed at the police station.

Inside the school, an older man wearing a light blue robe appeared, his eyes puffy from beatings and blood dripping from his mouth. He was carried by apologetic men - interrogators who had mistaken him for a member of the pro-Assad shabbiha militia.

A rebel spoke of another captive, believed to be from the shabbiha. "Even I got to step on him," he said.

"He deserved to be killed. He has raped women and shot protesters. Everyone in Aleppo hated him and we've finally got our hands on him. For sure he is going to be killed"

Asked if he would be killed immediately, the rebel replied: "No, we will send him to be tried, but I can't imagine he won't get the death sentence".