Inmates took over a prison farm on Sunday, seizing guards and visitors as hostages, according to news reports.
TV station Notisiete broadcast unconfirmed reports that five people were killed--three guards and two inmates.
Firefighters and Red Cross workers reportedly were seen entering the prison with five stretchers.
The station said Easter was a visiting day and that about 250 prisoners were holding an estimated 400 people hostage, including guards, other inmates and visiting families, among them women and children.
It did not say what set off the uprising at the Pavon prison farm, 12 miles east of Guatemala City.
Notisiete said that national police were called in and that additional army units were on standby.
Other reports said officials had tried to bargain with prisoners to get them to surrender rifles they grabbed from prison guards. After a shoot-out, most of the rifles were recovered. Negotiations were under way, according to the reports.