Emboldened by the vast reduction in fighting brought about by Syria's month-old ceasefire, civilians in dozens of rebel-held areas have taken to the now somewhat safer streets to demand democracy. On March 11, crowds assembled in Maarat al-Numaan, a town in northwestern Idlib province, for the second week running, brandishing the three-starred revolutionary flag of the Free Syrian Army (FSA) and chanting for the downfall of President Bashar al-Assad's regime.

They were quickly met with an angry response. But instead of pro-government forces, the objection came from local al-Qaeda affiliate Jabhat al-Nusra, which dispatched a mob of militants on motorbikes to disrupt the protest. The new arrivals attempted to drown out revolutionary slogans with shouts of Allahu Akbar ("God is the greatest") and when that failed, turned on the protesters themselves.

The clashes, which led to more defiant counterdemonstrations, were an illustration of how seriously tensions have risen between the Western-backed secular FSA and the hardline religious group Jabhat al-Nusra — tensions which could evolve into broader hostilities, representatives from both Nusra and the FSA told VICE News.

The recent demonstrations marked a return to the peaceful origins of 2011's popular uprising; now largely obscured by an increasingly sectarian conflict and growing strength of hardline opposition factions. But they also posed a direct challenge to Nusra's salafist-jihadist beliefs and grip on power. Now, in its attempt to quash them, Nusra may have gone too far.

Ahmad al-Saoud, leader of the Maarat al-Numan-based FSA's Division 13, played a prominent part in protests in the town. Like the rest of Idlib, Maarat al-Numan is largely controlled by Nusra, which is classed as a terrorist group and so not included in the Syrian truce, alongside its similarly hardline allies.

Saoud's brigade meanwhile, which has battled both Assad's troops and the Islamic State (IS) group, say they're fighting for a secular, democratic Syria, and have received training and support from the CIA, as well as deadly US-made TOW missiles from the "Friends of Syria" coalition. It enjoys a broad base of local support too, and regularly coordinates humanitarian operations in the region.

Clashes broke out during a pro-democracy protest on March 11 (seen at 01:52)

Nusra attacked Division 13 a day after the March 11 demonstration, overrunning its headquarters and a number of weapons depots as well as killing six of Saoud's men and capturing several more. It did not take possession of any TOWs, however, Saoud told VICE News in a recent Skype conversation. The commander said the Nusra attack was unprovoked, while the jihadists described the strike as retaliation after Division 13 detained one of its members.

Nusra has targeted Western-backed FSA groups before, routing the Syrian Revolutionaries Front (SRF) from Idlib in November 2014 and then forcing CIA-backed Harakat Hazm to disband in March 2015. However neither faction necessarily enjoyed widespread support either from the rest of the opposition or civilian populations in areas they controlled.

'Nusra were scared and they wanted to kick all of the people who have support from the West out of the area'

Division 13 was different, and has proven to be both militarily capable and popular among locals. In the aftermath of the clashes, civilians in Maarat al-Numaan began a series of anti-Nusra protests, demanding it leave town and eventually storming a Nusra compound to free the imprisoned Division 13 fighters. Nusra has since withdrawn to the outskirts.

The significance of these events may reach beyond one town. Al-Qaeda was able to establish Nusra as a major presence in Syria because its strength and funding made it one of the most effective anti-Assad fighting groups. It's since been adept at dividing and conquering potential threats, while forging relationships with factions of various stripes. Opposition groups were essentially forced into reliance or at least acceptance, whether or not they bought into its hardline views.

But the pause in fighting and resurgence of popular protest and public support for the FSA has sidelined it to a degree, exposing what could be a major weakness and demonstrating that its ideology may not ever have been widely accepted.

Watch the VICE News documentary: Inside the Battle: Al Nusra-Al Qaeda in Syria:

Indeed, in Maarrat al-Numan tensions between Nusra and Division 13 had been building for some time, including over Saoud's men's refusal to abide by what Nusra saw as Islamic norms; they played music and allowed women to wear makeup, for instance. "The relationship between us has for a long time been bad, in reality there never really was a 'relationship' between us," the FSA commander said.

Nusra finally had enough. A member of the group's media team whose family members had been involved in the clashes told VICE News via Skype that while a militant had indeed been detained by Division 13 fighters, this was a "normal problem," which would usually have been solved through negotiation. After the protests, however, it provided an excuse for a larger assault. "People in Nusra wanted the attack, like what happened with Harakat Hazm. They wanted a reason to attack them," he said.

'Nusra turned on [the Syrian people] and betrayed us, that was an injustice.'

The source, who spoke on the condition of anonymity, added that the al-Qaeda proxy had been seeking a pretext to go after foreign-backed forces for some time. "There was a reason to fix this without an attack. But Nusra were scared and they wanted to kick all of the people who have support from the West out of the area," he said.

Both sides see this as a precursor to further hostilities. Saoud said his group had now decided to "end [the presence of] Nusra in Syria". "They cannot be present because their ideology is outside the mindset of the Syrian people," he said. "Nusra turned on [the Syrian people] and betrayed us, that was an injustice."

Taking on Nusra will not be possible alone, and while Division 13 initially complained in a statement of a "strange silence" from other FSA groups regarding the clashes, Saoud said there was now a broader movement against the jihadists.

"There was definitely silence and betrayal from the factions of the FSA (leadership) and from the other revolutionary factions, but the protests woke up the factions and all the revolutionaries to this crisis," he said. "All the people and revolutionaries took note of what happened, they saw how the people had changed the equation and how they forced Nusra from Maarat al-Numaan."

An alarmed Nusra, concerned by recent events and what they see as a broader resurgence in the power of some FSA factions, will soon announce the formation of a new brigade in conjunction with ally Jund al-Aqsa dedicated to battling the FSA and other opposition groups, the Nusra source said.

He expected these preparations to spill over into heavier fighting "very soon" and said Nusra was preparing bases in Hama province and elsewhere in case it was forced to pull back.

'Within the Islamist rebel sphere people have increasingly had enough [of Nusra]'

Noah Bonsey, a Syria analyst with International Crisis Group, described Nusra's actions in Maarat al-Numaan as one of the more serious in a series of infractions, which in some ways resembled those that provoked a coalition of rebel groups to turn on IS in 2013. "You're seeing a consensus building amongst armed opposition and activists to stand up to what's seen as a growing Nusra problem," he said. "This is the latest overstep, but it follows a number of other incidents elsewhere in the country."

He added however that in previous clashes between rebels and jihadist groups, the opposition has only been successful if they've acted together, and even a cohesive FSA-only effort is unlikely to be successful. Key to ousting Nusra would be the addition of Islamist elements, namely powerful Turkish-backed militants Ahrar ash-Sham. Its members are widely seen as holding similar beliefs to Nusra, but more recently, its leaders have been attempting to rehabilitate themselves in the eyes of Western leaders, even writing opinion pieces in the Washington Post and Daily Telegraph.

The desire among at least some of Ahrar ash-Sham's commanders to be seen as a potential partner for Washington and its allies in taking on IS and Assad could turn it against Nusra, Bonsey suggests. "Within the Islamist rebel sphere people have increasingly had enough [of Nusra]," he said. "It's very clear that there's a continuum within Ahrar ash-Sham that are increasingly fed up with what they see as systematically problematic behavior from them. Whether it's enough to take them on board [with the FSA] remains to be seen."

Ahrar ash-Sham has thus far avoided public involvement in the spat and is understood to be attempting diplomatic efforts between Nusra and the FSA. But senior leader Hossam Salamar spoke at what essentially amounted to anti-Nusra protests in Maarat al-Numaan on Friday, a potentially significant development.

'The Syrian people will not accept al-Qaeda in Syria, whatever the cost'

The Nusra source said the leadership was also worried about Ahrar ash-Sham. He predicted that a joint FSA alliance would turn on Nusra before being joined by Ahrar ash-Sham and Islamist rebel alliance Faylaq al-Sham. He added that because many of Ahrar ash-Sham's members held essentially the same salafist-jihadist beliefs as Nusra, there might well be widespread defections under this scenario. "Here Ahrar ash-Sham will have no option but to split. Part of them will join Nusra, some will join FSA, and others will leave their weapons without a fight."

Protests took place elsewhere in Idlib again last weekend, with crowds calling for Nusra's exit. Saoud from Division 13 urged rebel groups and the Western powers that back them to take the opportunity to counteract al-Qaeda in Syria. "We are trying to continue [in our fight against Nusra] until the anger of the people against them [is made known], and forces a military decision to expel them because they don't have the support of the people," he said. "The most important thing to understand in the Syrian revolution... is that the Syrian people will not accept al-Qaeda in Syria, whatever the cost. Bashar [al-Assad] will not fall until al-Qaeda and others like it fall first. We in Syria completely reject the ideology of al-Qaeda. We are a moderate Muslim people, we are open-minded and we love our country and want to live in peace and security."