(CNN) It's not hard to get into Raqqa. The problem, one prominent activist tells CNN, is getting out.

Abu Ibrahim al-Raqqawi's voice is calm and measured as he describes life inside the city ISIS claims as the capital of its so-called caliphate. But the horrors he details are harrowing.

Airstrikes. Executions. Forced blood donations and marriages to ISIS fighters.

Al-Raqqawi isn't his real name. It's the identity the former medical student who helped found an activist group called "Raqqa is Being Slaughtered Silently" uses to speak out. ISIS fighters have already tortured and executed one member of the group, he says, and they've made it clear they want the others involved dead.

Still, al-Raqqawi tells CNN's Brooke Baldwin that won't stop him from sharing what he sees.

"I lost my life ... I don't have school. I don't have a future. I don't have anything, but I didn't want that for me or for my city," he says. "It's the situation forcing me to do this. I don't want to be famous. I don't want everybody to know who I am or what I'm doing. It's just for my city and for my family and for the innocent civilians. ... We are trying our best. We are trying to save our city."

Here's a look at some of the things al-Raqqawi has told CNN he and others from the activist group have seen in Raqqa. CNN hasn't independently confirmed his claims, but his account provides a rare glimpse into the dramatic transformation of a city once known as one of Syria's most liberal

Dozens executed

In two months at least 40 people have been executed in Raqqa, al-Raqqawi says, for charges including being a fighter for the Free Syrian Army, being gay or killing others. Activists, he says, are also targeted.

"If you are an activist...inside the city of Raqqa, it will take you to death," he says.

Forced blood donation

In Raqqa, al-Raqqawi says, a day might start out with a trip to court over a home robbery, and end up with a forced blood donation.

"If you have anything in the Islamic court, they say, 'Go to the hospital and donate your blood and bring me the receipt, and if you don't have it, I can't help you," he says. "They say to you, 'We can't help you until you go to the hospital. There (are) a lot of airstrikes, a lot of injured ISIS fighters.'"

Women forced into marriages

For women, al-Raqqawi says, the city is "like a big prison." They are not allowed to leave the city if they are younger than 45 years old. And he says his activist group has documented more than 270 cases of girls forced to marry ISIS fighters.

"ISIS fighters are really sex-mad. ... Some of them have two and three wives, and even with that they are trying to find slaves from Yazidi girls," he says.

Buildings get an unexpected facelift

An effort to repaint buildings in bright colors isn't what you'd expect to see in a square known as the site of executions, beheadings and crucifixions. But al-Raqqawi says that's just what happened in Raqqa several weeks ago.

The change is particularly notable, he says, because when ISIS first came to the city they painted buildings black. Now, al-Raqqawi says the fresh coats of pink, white, gold and green paint could be a sign that ISIS is trying to make buildings less of a target for coalition airstrikes.

Foreign fighters flood city

"There is a big wall between the civilians and foreign fighters. It's like two different lives inside the city of Raqqa," al-Raqqawi says. "Yes it's heaven for some of these foreign fighters, because they give them a lot of money. They give them the fancy houses. They give them the fancy cars."

But for some, it's not the paradise they imagined, al-Raqqawi says. Rumors swirl, he says, about foreign fighters being killed after trying to defect.

"ISIS takes their passports and if anyone tries defection from this, they will kill them immediately," he says. "The problem, it's not how to go inside the city of Raqqa. The problem is how to get out."