Dozens of detainees died in a riot in the northern Mexico city of Monterrey on Thursday after rival groups joined in a "pitched battle" in an overcrowded prison.

Based on a CNN report, the riot began as a late-night bout between leaders of rival groups and grew into a massive prison-wide riot that left at least 49 people dead and 12 others injured, five of which are in critical condition.

According to Nuevo Leon state Governor Jaime Rodriguez, the inmates used bats, sharp weapons and sticks during the brawl that lasted for about 30 to 40 minutes and ended at about 1:30 AM on Thursday.

It lasted that long due to a fire that had reportedly begun in one of the supply rooms which was ignited by some of the prisoners involved in the riot.

According to Reuters, the brawl involved members of the notorious drug cartel known as the "Zeta 27" and their rivals at the old and highly populated Topo Chico prison in Monterrey, Mexico.

Authorities told BBC News that the conflict between Juan Pedro Zaldivar Farias of the Zetas 27 and Jorge Ivan Hernandez Cantu of their rival Gulf cartel sparked the deadly riot which is now being dubbed as one of the worst in the country.

"Topo Chico is a ... very old prison. A prison with very difficult security conditions," Rodriguez explained.

According to AFP as posted in Tico Times, the 60-year-old prison accommodates a total of 3,800 inmates which is twice its actual capacity.

"We are experiencing a tragedy stemming from the difficult situation that they are living through at penitentiary facilities," the Nuevo Leon state governor explained.

He further assured that they are doing everything they can to determine the identities of the casualties of the riot. 40 out of 49 of the victims have already been identified.

Relatives of inmates remain uneasy as some reportedly threw sticks and rocks while trying to pry open the gates to the establishment.

In an interview with BBC News, Ernestina Grimaldo, a mother of an inmate, claims that they were not informed of anything since they got there.

"They said that until there is order they won't let us in. Everything is in disorder, and nobody is telling us anything," she told BBC.

In a separate interview with AFP, Mrs. Grimaldo said she and the other relatives of the inmates will remain in the area until authorities provide them with information about their loved ones.

After the incident, some 20 inmates were evacuated per a statement from an unnamed state official.