The killing spree in the town, which has become a symbol of Kurdish resistance, was widely seen as vengeance for a series of defeats inflicted on the jihadists by Kurdish militia in recent weeks.
It began after three IS suicide bombers blew up vehicles at the entrances to the town yesterday clearing the way for fellow jihadists to enter.
Women and children were among the slain civilians whose bodies were found in their homes and in the streets, the Syrian Observatory for Human Rights said.
"According to medical sources and Kobane residents, 120 civilians were executed by IS in their homes or killed by the group's rockets or snipers," Observatory director Rami Abdel Rahman said.
"When they entered the town, the jihadists took up positions in buildings at the southeast and southwest entrances, firing at everything that moved."
Local journalist Mostafa Ali said that there was no military dimension to the IS assault on Kobane.
"IS doesn't want to take over the town. They just came to kill the highest number of civilians in the ugliest ways possible," he said.
The jihadist group also executed 26 civilians in a village outside Kobane yesterday, the Observatory said.
"Every family in Kobane lost a family member on Thursday," Kurdish activist Arin Shekhmos said.
The jihadists had entered Kobane at dawn yesterday disguised as fighters of the Kurdish People's Protection Units (YPG), said journalist Ali.

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