Syria crisis: a captured Shabiha member in Aleppo speaks Televisió de Catalunya

The green tanks of Bashar al-Assad's army twinkled in the afternoon haze – two of them, parked just outside a military base. From the Syrian rebel position, a mere 1.5km away, an opposition fighter peered curiously at them. He poked his binoculars through a tiny window, gesturing to watch out for snipers.

At that moment there was a percussive boom: a shell landing in the nearby hills. Syria's war has been going on for 16 long months – a brutal conflict fought between a well-armed military state and lightly-weaponed revolutionaries. This battle has raged across the country: in Homs, Hama and most recently Damascus.

Last week it arrived in Aleppo in northern Syria. It is the country's biggest city, home to 2.5 million people, inhabited since the second millennium BC, and situated on a historic trading route between the Mediterranean and Mesopotamia. It is a microcosm of Syria's complex internal forces, religious and civic. A murderous storm now grips it.

"The bombing has been going on all night. And all morning," Abdul Sadiq, a Free Syrian Army commander, said indifferently. "The shelling is continuous." His militia volunteers seized the town of Anadan, 13km immediately north of Aleppo, a month ago. Groups of FSA fighters infiltrated south and east Aleppo on 20 July.

Over the past week Damascus has responded by pulverising rebel-held districts using artillery and helicopter gunships. A tank column has arrived in the western suburbs. In the meantime, tens of thousands of civilians have fled – believing that Aleppo's fate is likely to be darkly similar to that of Homs, the cradle of Syria's revolution, much of it now a smouldering ruin.

Sadiq's rebels have taken over a florid Italian-style villa in Anadan, until recently occupied by regime officers. The Syrian army is encamped within jogging distance in the nearby Hraytan base; since pulling out it has targeted the building's new owners relentlessly. Large chunks have been gnawed from the villa's yellow balustrades. The murky swimming pool round the back is untouched; two cats were mewing on Saturday in the ornate gardens.

Despite the unpromising facts on the ground, the rebels were upbeat. "The [regime] army is sick. It is destroyed from inside. There is no power inside it," Abu Ahmed, another commander, explained. Striking a vivid analogy, he added: "They have plenty of weapons from Russia. But this is like an injection into the arm of a dying man." One fighter then showed off a large mortar that had landed on the terrace. "Don't worry. It didn't explode," he said.

Anadan was once a holiday resort used by wealthy Aleppines, its mountains somewhat cooler than the city. The town now lies abandoned. Concrete buildings have been destroyed, shells have made wonky the electricity pylons and the town's brown fields bear the pitted scars of nightly mortar attacks. Only a handful of grizzled revolutionaries with Kalashnikovs exist here.

The surrounding landscape is an ancient kaleidoscope of ghostly Byzantine churches and poor Kurdish hamlets. In the mountains close to here lived the fifth century saint St Simeon the Stylite; a remarkable cruciform church marks the spot where he spent many years preaching from a column. Aleppo is still home to many Christians of various denominations, most fearful of what their future might be in a post-Assad Syria.

Others, however, have enthusiastically embraced the revolution. In the opposition-held town of Dar Ta'zar, several students from Aleppo University were now with the local FSA. Many of them said they had fled their campus following a campaign of terror this spring by regime security forces. In February troops attacked students who had been demonstrating against the Assad regime, killing at least four of them. One was shot in the head and two in the chest, witnesses said.

"[The university] was like a big jail, actually. A lot of students were being arrested by the secret police," Saeed, an engineering student, 23, recounted on Saturday. (He declined, for understandable reasons, to give his second name.) "We stopped going to classes. You couldn't walk in the city. If you were near a demonstration they would capture you."

Another student, Amar Naser, also 23, said the security forces first stormed the university last October, arresting 500 people, ordering them to lie on the ground and then beating and kicking them. Troops returned in early March, he said, throwing one student to his death from a sixth-story dormitory. One student, Mohamad Asaad, said he witnessed this – a war against the country's intelligentsia.

Naser said: "They treated us like we were the enemy. They would stop anyone carrying a laptop. It was like: 'Oh, he's a terrorist! Get him.' They would ask: 'Do you have Facebook? Where did you buy Facebook?' The Shabiha [regime paramilitary forces] are so stupid they don't even know what Facebook is."

Naser said the secret police arrested him when he was halfway through his final engineering exams, with four papers to go – meaning that he failed the year. Typically, the regime swept up boys, he said, but also detained a few girls, with some better-off parents able to buy their offspring out of jail for sums between $200 (£127) and $20,000.

As a Sunni, he said, he had no prospect under the current regime of leaving the country or fulfilling his long-term dream to be an architect in Germany. He said he was worried about one of his friends, Mohamad Hussein, a third-year engineer. Hussein had disappeared: "I don't know where he is now. Maybe he is dead. Maybe he is lost."

The students – one of whom said he was studying philosophy and liked Plato – were resting with other fighters in a basement FSA HQ. Two rebels then came in escorting a prisoner. He turned out to be a member of the Shabiha militia captured during fighting inside Aleppo on Wednesday.

The prisoner identified himself as Dawish Dado, 33. He said he was a decorator in Aleppo, and was recruited two months ago to join the Shabiha as the regime's grip on the country began to unravel. Dado was in handcuffs; he had two black eyes and a plaster over his possibly broken nose. "I got these fighting with the FSA," he explained diplomatically squatting in the corner.

Asked if he killed anyone himself, Dado replied: "No." But he confirmed his Shabiha unit had looted many houses in Aleppo, robbed people, and probably raped women. "I didn't witness rape. But I heard talk about it in my unit," he said. What did they steal? "Private property. Pretty much everything we could lay our hands on," he indicated.

The Shabiha included some criminals freed from jail by Assad, as well as opportunists, out for whatever they could get, he suggested. Unlike the army, which has seen mass desertions by Sunnis, the Shabiha was a mixed force, he said. His unit included Sunnis as well as Alawites – Assad's minority ruling sect – and worked, he suggested, as the regime's thuggish enforcers.

"They Shabiha don't want the regime. They don't want the revolution either. They simply want to destroy the country," he said candidly, sitting in an FSA cell. He said a colonel from airforce intelligence, Abdul Latif, had recruited him. Latif told him he was fighting against a "terrorist group" and offered him a salary of 20,000 dinars a month ($300).

Residents inside Aleppo, meanwhile, say not everybody in the city supports the revolution, with the wealthy viewing the rebels as a sort of unwelcome peasant army. "If I were to generalise I would say the middle class and upper class don't want the rebels. They want everything to be how it was so they can trade and go to coffee shops," one English-speaking resident, who lives in a regime area, said via Skype.

The FSA fighters who slipped into the city nine days ago now control a crescent-shaped chunk of Aleppo, a city known to Syrians by its Arabic name of Haleb. They moved into districts where they are confident of support, setting up checkpoints bordering other areas possibly more sympathetic to the regime. They are armed with rocket-propelled grenades and Kalashnikovs – not enough, it would appear, to defeat Assad's tanks and helicopter gunships.

Assad's strategy seems clear enough: to besiege the rebels inside Aleppo, as in Homs, and to shell them until they are crushed. The difference now, however, is that the FSA controls large chunks of the Syrian countryside, including the environs of Aleppo. And it believes it is winning. "They think they are besieging us. In fact we are besieging them," the commander Abu Ahmed said.