Concern over the fate of hundreds who have gone missing in a Syrian city run by the Islamic State of Iraq and the Levant has fueled a backlash against the jihadist group.

Mothers of the detainees gather every day outside ISIL bases in the city of Raqa, where the extremist group is in full control and governs through a harsh interpretation of Islamic sharia law, according to residents and activists.

"They cry, begging for information and for their sons' release," said Amer Matar, whose citizen journalist brother Mohammad Nour has been detained by ISIL for nine months.

"My mother suffers every day, because she is not given any information about her youngest child," said Matar, a filmmaker from Raqa who became a refugee in Germany because of his own activism against President Bashar Assad.

The kidnappings and other abuses led activists to mobilize a new campaign against ISIL last week that has gathered support on social media networks and seen protests held across opposition-run areas.

Rooted in Al-Qaida in Iraq, ISIL first emerged in Syria's war in spring last year, and was initially seen as an ally by fighters seeking Assad's overthrow.

But ISIL's quest for hegemony and systematic abuses -- including the kidnapping of hundreds of rebels, activists and even ordinary citizens accused of "crimes" like heresy and smoking cigarettes -- eventually turned the opposition against the extremist group.

ISIL has in recent months been expelled by rival rebels from many opposition areas, but the northeastern city of Raqa on the Euphrates river remains squarely under its control.

Sema Nassar, a prominent human rights activist, says ISIL is believed to be holding "more than 1,000 Syrians in Raqa province, though it is impossible to know the exact number."

She also said those suspected of opposing ISIL or violating its puritanical social code vanish, all too often without a trace, while others have been publicly executed.

The province is home to an unknown number of detention facilities, including secret prisons where torture is especially severe, says Nassar, who works with the Syrian Network for Human Rights.

Activists describe a climate of fear not unlike that under the Assad regime, and many have fled to nearby Turkey, fearing arrest or worse.

"ISIL sees activists as a challenge to their power, who must be eliminated," said Nassar.

Despite the dangers, a group of dissidents using secret identities last week launched a campaign calling on ISIL to leave Raqa under the name “Raqa is Being Slaughtered Silently”.

Protesters across opposition areas last Friday adopted the slogan: "Cleansing Raqa of (ISIL chief Abu Bakr) al-Baghdadi's Gang" and on Facebook and Twitter activists share photos of the group's abuses.

One shows a field execution of several men, who kneel blindfolded in a public square, while another shows a man who has already been executed, tied to a makeshift cross in front of wide-eyed children.

Other groups report incidents including a woman given 40 lashes for failing to veil her face.

"It's extremely dangerous to oppose ISIL... but we need to break the wall of fear," said Abu Ibrahim, a campaign organizer.

"We must make sacrifices, or else they will rule us for good, and that's just unacceptable," he told AFP via the Internet from Raqa city.

The campaign has already raised ISIL's ire, prompting the arrest of some 70 people in Raqa in the past week alone, said Nassar.

"They've arrested anyone they've caught even opening Facebook for entertainment, people who aren't political at all. They've imposed some crazy version of emergency law on Raqa," she told AFP.

The opposition has frequently accused ISIL of colluding with the Assad regime, which since the start of the 2011 uprising has accused the entire opposition of being "terrorists" backed by foreign powers.

An army source told AFP the army is not interested in attacking ISIL because the government wants to make an example out of Raqa.

"We want the people to see what happens when the rebels take over," the source said.

While abuses are committed by all parties in Syria's war, activists say ISIL stands out for its brutality.

Human Rights Watch researcher Lama Fakih said her group is advocating measures to block financial support to ISIL, and to ensure the situation in Syria as a whole is referred to the International Criminal Court.

"Such a referral would enable some victims to achieve justice, and also act as a deterrent for the future," said Fakih.

But relatives of the kidnapped are not hopeful.

"We are ready to do whatever it takes to secure Mohammad Nour's release, but ISIL haven't even admitted they have him," said Matar.

"We feel paralyzed. We Syrians are like citizens of nowhere, and can go to no one -- no state or power -- to ask for support."