Clashes between guards and prisoners at a jail in Syria have resulted in many deaths, human-rights groups have said.
At least 25 people were killed after military police fired live bullets at Islamist inmates, the groups said.
The Syrian authorities have so far not commented on the situation. Prisoners said the clashes were sparked by raids in which guards beat inmates.
One inmate told the BBC he believed the death toll was higher. The prisoners are reportedly holding hostages.
Several prisoners have managed to contact Syrian human rights group, as well as the BBC, by telephone.
They said the guards had also desecrated copies of the Koran.
The inmates said the early-morning raids were in response to a protest by detainees several weeks ago about conditions at Saydnaya Prison near Damascus, which houses chiefly Islamist and political prisoners.
One inmate told the BBC the guards had roughly treated the prisoners during the raid.
"They shackled our hands behind us, confiscated our clothes and possessions, and beat us. And they insulted the Koran, they trod on the Koran," he told the BBC's Arabic service.
The Syrian Observatory for Human Rights quoted a political prisoner reached by mobile phone inside the jail as saying that the riot had been started by Islamist inmates.
A number of prisoners had climbed on to the roof of the prison to escape continued shooting with live ammunition by guards, the group said on its website.
"The Syrian Observatory for Human Rights demands that Syrian President Bashar al-Assad intervenes immediately to stop this massacre," it said.
The group told the AFP news agency that several hundred soldiers were being held hostage in the jail in order to put pressure on the Syrian authorities.
Ammar al-Qurabi, of Syria's National Organisation for Human Rights (NOHR), told al-Arabiya TV, a Dubai-based channel, that it was too early to get a clear figure of how many had been killed or injured.
He said the authorities had viewed disobedience by the prisoners as rioting and so had responded with live ammunition.
The NOHR hoped the incident would push Syrian officials to listen to detainees' complaints over conditions, he said, rather than using force.
Muhammad al-Hassani, also from the NOHR, told the Associated Press news agency he could see smoke billowing from the building and prisoners standing on the roof.
He said ambulances were carrying the injured to hospital. Meanwhile, Syrian troops had closed roads around the prison, he said, and phone lines in the area had been cut off.