Armed sharia police patrolled the streets as "enemies" of the regime were crucified or decapitated, their severed heads impaled on spikes in the city square.
Student Abdalaziz Alhamza and his friends decided to form Raqa is Being Silently Slaughtered (RBSS), a band of courageous citizen journalists who risk their lives to document IS atrocities.
Their work is chronicled in "City of Ghosts," by Oscar-nominated director Matthew Heineman, one of a raft of films on conflict and jihadist terror in the Middle East that premiered this week at the annual Sundance Film Festival.
"So often in documentaries, subjects become caricatures of whatever they're doing in life. For me, that's not very interesting," Heineman, 33, told AFP.
"I very much wanted to spend as much time as possible to understand who these guys are, what makes them tick, what are their emotions, feelings and thoughts.
"Until I get to that level of intimacy, I don't feel like I have a film."
No journalist has been able to enter the region, allowing IS -- also referred to as ISIS -- to control the message about what is happening in Raqa via increasingly slick videos.
While IS paints a picture of a fully-functioning, prosperous city, RBSS has been able to capture the shocking brutality and dysfunction of everyday life in the city of one million.
Following a lightning offensive in which IS was accused of numerous atrocities, the group declared its caliphate stretching from northern Syria to parts of eastern Iraq in June 2014.
Alhamza, 25, first encountered the group when a masked man with a Saudi accent burst into his university and recruited one of his friends, who later turned up dead.
RBSS documents the atrocities committed daily by the extremists on camera phones, smuggling encrypted footage via the internet to Alhamza and his fellow exiles, who disseminate it via social media.
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Last year, they reported to the outside world on a 20-year-old jihadi who had shot his mother in the head with an assault rifle in front of a crowd, after she was accused of apostasy.
Heineman was touring America with his Oscar-nominated 2015 Mexican drug trade documentary "Cartel Land" as the plight of Syrians was becoming a near-daily part of the news cycle.
He began researching the conflict extensively and came across RBSS in the fall of 2015, and was struck by the sacrifices that its members had made.
He decided early on that he wanted the core of the story to be deeply personal "verite" footage, captured as the activists escaped Syria after the assassination of several members by IS fighters.
Heineman followed them in Turkey and then eventually to Germany as the IS group continued to threaten them.
"I knew I wanted to juxtapose this journey with the amazing footage that they had from inside Raqa to show life under the caliphate, the formation of the capital of ISIS," Heineman said.
Heineman's story starts in Raqa but evolves into a rare human take on Europe's migrant crisis, as well as a moving chronicle of brotherhood and coping with trauma.
Elsewhere at Sundance, "Last Men in Aleppo," a documentary on the Syrian city's "White Helmet" first responders, also got its premiere.
A collaboration between Syrian filmmaker Firas Fayyad, Danish filmmaker Steen Johannessen, and the Aleppo Media Center, it follows three reluctant heroes who rush toward bomb sites while others run away.
"Cries from Syria," a third film on the crisis making its debut, tells how the country's people, inspired by events in Tunisia, Libya and Egypt, rose up against the dictatorial rule of President Bashar al-Assad.
The Islamic State's influence has spread far beyond the Middle East, and last week the festival screened a startling film about the kidnapping of 276 girls from the Nigerian town of Chibok by IS-affiliated militants Boko Haram.
Documentary short "Waiting for Hassana," by first-time director Funa Maduka, tells the story of the abductions from the perspective of one of 57 girls who managed to escape.
"We know the global story, now we hear the personal one," said Maduka, who also worked on 2013 Nigerian civil war picture "Half of a Yellow Sun."
"As the director, my aim was to visually and sonically plunge audiences into the psychological and emotional landscape of our subject."