BEIRUT (AFP) - The Islamic State in Iraq and Syria extremist group Saturday demolished a prison in the Syrian city of Palmyra, a monitor said, eliminating what was for decades one of the country's most feared detention centres.

The jail was "largely destroyed after ISIS planted explosives inside and around it", 10 days after the militants had seized Palmyra from regime forces, the Syrian Observatory for Human Rights said.

ISIS supporters posted pictures on Twitter purporting to show the infamous prison being blown up.

The prison was the site of a massacre in 1980 in which hundreds of inmates were killed.

It became notorious throughout Syria as a symbol of the brutality of the regime of former president Hafez al-Assad and his son and successor, Bashar.

Housing political prisoners until the 2011 uprising, Palmyra's jail later became overcrowded with regime deserters and draft evaders as peaceful anti-government protests morphed into a brutal civil war.

Regime opponents praised the jail's destruction on social media.

"ISIS has wiped out evidence of the crimes of the Assad clan by blowing up the infamous Palmyra prison," said Syrian opposition member Mohammad Sarmini on Twitter.

"Palmyra prison bears witness to the crimes of the century," an anti-Assad activist tweeted.

The Observatory said government forces relocated inmates held at the jail before ISIS overran the historic city of Palmyra, which houses priceless UNESCO-listed ruins and ancient artefacts.