The hardline Sunni militants took control of the central city, also known as Tadmur, from government forces last week and have killed at least 200 people and taken around 600 captive, according to the Observatory.
“They executed around 20 men in the Roman amphitheatre and called people to watch,” said the Observatory’s Rami Abdulrahman, citing sources inside the city.
Supporters of Islamic State wrote on Twitter that a number of people had been killed by the group inside the amphitheatre, which forms part of the city’s 2,000-year-old ruins which are a UNESCO World Heritage site.
Islamic State’s takeover of Palmyra marked the first time the group had seized a Syrian city directly from government control. The other population centers it holds were mostly taken from rival insurgent groups in Syria’s four-year conflict.