The purge is part of a clampdown by the extremist group to solidify its grip on the town since overrunning it late on Wednesday.
The militants have also imposed a curfew from 5pm until sunrise and banned people from leaving town until tomorrow morning to ensure none of the government figures they seek manage to escape, activists and officials said.
The door-to-door hunt for opponents is similar to a purge the militants carried out in the Iraqi city of Ramadi after capturing it last week.
"The search is going from house to house, shop to shop and people on the streets have to show identity cards," said Osama al-Khatib, an activist from Palmyra who is in Turkey.
Mr al-Khatib last contacted his friends and relatives in Palmyra this morning before the government cut off all land and mobile telephones as well as internet service in the town.
IS fighters have also detained dozens of suspects after seizing Palmyra, which is home to one of the Middle East's most famous archaeological sites, activists and officials said.
Homs-based activist Bebars al-Talawy and an opposition Facebook page said that as many as 280 soldiers and pro-government militiamen have been killed in Palmyra since it was captured on Wednesday.
Mr al-Talawy said militants abducted soldiers and pro-government gunmen from homes, shops and other places where they had sought to hide. He added that many were shot dead in the streets.
He said IS fighters used loudspeakers to warn residents against sheltering troops, leading many to come forward to give information about forces who had melted into the civilian population.
Mr al-Khatib said 150 bodies lay in the streets of Palmyra, including 25 members of the pro-government militia known as the Popular Committees who were Palmyra residents.
Maamoun Abdulkarim, head of the Antiquities and Museum Department in the Syrian capital Damascus, said: "There are arrests and liquidations in Palmyra."
He added that IS fighters are "moving in residential areas, terrifying people and taking revenge".
Mr Abdulkarim said no gunmen were seen in the area of Palmyra's 2,000-year-old ruins, which once attracted thousands of tourists.
The Britain-based Syrian Observatory for Human Rights said IS fighters have killed 17 men in Palmyra and that it has unconfirmed reports of the killing of dozens more. The Local Co-ordination Committees said IS fighters have killed dozens of people since Wednesday, including three siblings, two teenage girls and a teenage boy.
Governor Talal Barazi of the central province of Homs, which includes Palmyra, said IS fighters have abducted men and "might have committed massacres". He added that about 1,400 families left the town of 65,000 before IS started preventing people from leaving yesterday.
A video posted on a pro-IS Facebook page showed residents and militants gathering around two bloodied men in military uniforms on a Palmyra street. "Let all the residents see them," one of the men in the gathering tells an IS fighter.
The Observatory and Mr al-Talawy said IS's next target appears to be the Tayfour air base near Palmyra, where many of the government troops retreated. They said IS was sending reinforcements to the air base area.
Mr al-Talawy and Mr al-Khatib said IS had also captured over the past day the phosphate mines at Khunayfis, near Palmyra.
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