Two Danes lie in the back of a pickup truck on the Turkish-Syrian border waiting for a call. They're with a young British jihadi and a group of ultra-conservative Syrian Salafi Islamists. Soon they'll be smuggled into Syria to join the fight against the Assad regime.

One of the Danes, a 40-year-old father of three, doesn't want to be named. The other doesn't care; he knows that as long as he avoids proscribed terrorist groups, he can't be prosecuted under Danish law. On the streets of Copenhagen he is known as Big A, a convicted drug trafficker and the leader of one of Denmark's most notorious organised crime gangs. But on the frontline in northern Syria, Abderozzak Benarabe is just another have-a-go jihadi, a man who joined the fight against Bashar al-Assad through a mix of restlessness, curiosity and a need for some kind of personal redemption.

I followed Benarabe's experiences in northern Syria and subsequently back in Copenhagen, for a documentary broadcast by the Guardian on Tuesday. The footage was shot in 2012, but the scenes it exposes take on added resonance in the light of mounting concerns about the number of Europeans believed to be flocking to Syria. Some estimates put the number at about 1,000.

It's not all plain sailing for the Dane. Benarabe's entry into Syria is secured by crawling through a hole in the Turkey-Syria border fence and running through fields for 10 minutes until he and his fellow travellers reach a waiting car. They are driven to Sarjeh, a city in the Zawiyah mountain region of Idlib, north-west Syria.

There, they meet a burly Canadian of Iraqi descent with a long beard and military fatigues. "God willing, we'll replace Assad with an Islamic state. I am tired of the immorality in Canada and if we succeed I'm going to live here," he said.

Also there are three Uzbeks. Two of them speak neither Arabic nor English, and have to communicate with the rebels with hand gestures.

Every morning, rebel fighters leave for a battle that's raging in the nearby city of Ariha. Benarabe endures a frustrating induction during which he isn't even given a weapon. He befriends a group of young fighters – they say they're 16, but look younger – and hangs out and shares cigarettes with them, and borrows one of their AK-47s to fire on an enemy target. He's clearly familiar with guns and needs no instruction on how to shoot.

But the sights of the battlefield are grim even for a man inured to violence. A machine-gunner gets shot in the face, just yards from where I am standing. The bullet penetrates his skull and hits another rebel behind him in the chest. Two more fighters are hit as they fire off rounds at the sniper. I follow them into the makeshift operating theatre – a bedroom in one of the houses – and film as two more rebels lie dying as doctors try in vain to perform CPR. Three are dead and their mourning comrades cover them with sheets.

The aerial bombardment intensifies and Benarabe and the rebels take shelter in the lower floor of the apartment blocks. The idea is that the bombs will not penetrate the upper floors. All they can do is wait and pray while the sound of the jets gets louder and the bombs fall. The blasts are getting closer. Suddenly a massive explosion shakes the building. Rubble falls, glass shatters and the air is thick with smoke and dust. It was very close.

The commander, Ibrahim Abu Muhammed – a small, softly spoken man with rosy cheeks – sits and reads his Qur'an.

"We're staying here and showing them that we are resilient," he says, looking his men in the eyes. "We'll show them we are only afraid of Allah."

That night, after they have been relieved by a contingent of fresh fighters, the other Dane starts to act strangely. He appears traumatised by the violence and becomes paranoid that the rebels want to kill him. He thinks my camera is making them nervous. To calm him down I stop filming, telling the commander my camera is broken.

For the next four days, Benarabe is at the forefront of the battle. He comes back one night with a grenade fragment that hit his backpack and his comrades tell of his bravery in leading charges and volunteering for every mission. The young fighters hope he will lead their unit, and Benarabe talks of buying a house in the town and funding and equipping the contingent himself. He has become even more fervent in his belief that he is fighting for a worthy cause. "We have to stop the Syrian army," he says, "Or we'll have more massacres like in Daraya."

But the commander quickly realises that Benarabe will be more useful to the rebel effort drumming up cash and supplies from Copenhagen.

Back in the Danish capital, Benarabe collects about £50,000 through donations and, it is rumoured, through "taxing" local drug dealers. He buys three mini-vans and fills them mostly with medical equipment, but he also says he's transporting some hi-tech military paraphernalia, including night-vision goggles and heat sensors.

He and some associates drive the cargo to Syria and hand it over to the rebels. Benarabe plans to stay and fight.

Big A, who leads one of Denmark's most notorious gangs, needs no instruction in how to use a weapon.

Then he drops off the radar. Soon afterwards, I hear he is back in Copenhagen. He never said why he didn't stay in Syria, but there was a flair-up in gang violence in Denmark at the time and there were rumours that rival gangsters were trying to muscle in on his turf.

I next catch up with him in Morocco, where he is hiding out from an aggravated assault charge. He is proud of what he's done in Syria, and still talks of being religious, but he says his loyalties to his gang have superseded anything else.

"These are my brothers, the people I grew up with, and when they need me I must do whatever I can for them."

Benarabe is currently in jail in Copenhagen. Last I heard, he was in hospital after being badly beaten by inmates from a rival gang.