Hadi Abdallah, a Syrian activist in the central city, told AFP the bodies of 26 children and 21 women were discovered in the Karm el-Zaytoun and Al-Adawiyeh neighbourhoods of the besieged central city.
"Some of the children had been hit with blunt objects on their head, one little girl was mutilated and some women were raped before being killed," he said, adding that most victims had been stabbed to death or had their throats cut.
Activists posted videos online showing graphic images of charred bodies and children with mutilated and bloodied faces.
Syrian state television attributed the killings to "armed terrorist gangs", saying they had kidnapped residents of Homs, killed them and then made video footage of the bodies in an attempt to discredit Syrian forces.
News of the killings sparked a mass exodus from the city, which has been besieged by Syrian forces for more than month, according to Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights.
"Hundreds of families fled Homs overnight, notably from the Karm el-Zaytoun neighbourhood, for fear of new massacres by regime forces," Abdel Rahman told AFP in Beirut.
The main opposition group, the Syrian National Council (SNC), called for an emergency UN Security Council meeting to discuss the "massacre", which it said took place on Sunday.
"The Syrian National Council is making the necessary contacts with all organisations and countries that are friends with the Syrian people for the UN Security Council to hold an emergency meeting," the SNC said in a statement.
And in a clear reference to Russia and China, the SNC said that allies of Syria's President Bashar al-Assad shared responsibilty for the "crimes" committed by his regime.
The latest killings in Homs came after UN-Arab League peace envoy Kofi Annan left Damascus on Sunday without managing to secure an accord to end bloodletting in Syria, where more than 8,500 people have died since the revolt against President Bashar al-Assad erupted a year ago according to the Syrian Observatory.
Annan departed at the end of a two-day mission during which he said he presented Assad with "concrete proposals" to halt the unrest that has rocked Syria since pro-democracy protesters rose up against his regime on March 15, 2011.
On the ground, more than 150 people -- 61 of them civilians caught in the crossfire -- were killed in weekend clashes between armed rebels and regular soldiers in various flashpoint areas, according to the Observatory.
Most of the deaths occurred in an ferocious assault by regime forces against rebel bastions in the northwestern Idlib province, it said, adding that fighting also occurred Sunday in the central city of Hama, the nearby province of Homs, and in the Damascus countryside.
Annan on his first mission to Syria to attempt to secure a halt to the violence, had emerged positive from talks on Sunday with Assad, a follow-up to their first meeting the previous day.
"It's going to be tough, it's going to be difficult, but we have to have hope. I am optimistic," Annan told reporters, while stressing the urgency of finding a solution.
"The situation is so bad and so dangerous that all of us cannot afford to fail," the former UN chief warned, in response to a suggestion that dialogue with the government was futile.
Assad had insisted during their first meeting on Saturday there would be no dialogue until the "terrorist groups" he claims are fomenting the violence are disbanded.
Opposition figures in their meeting with Annan however were adamant that the regime troops pressing the crackdown on dissent must first return to barracks before talks can begin.
Diplomats at the United Nations in New York had expressed pessimism about the prospects for Annan's mission after troops poured into Idlib city, which lies in the province by the same name, late on Saturday just hours after his first meeting with Assad.
Annan was in Doha on Monday to discuss the situation in Syria with Qatari officials, a government spokesman said without giving details.
The military crackdown in Idlib came after the Homs neighbourhood of Baba Amr was stormed on March 1 after a month-long blitz in which hundreds of people died.
China meanwhile on Monday welcomed a Russian and Arab League joint plan for ending the deadly violence in Syria as "positive", and renewed its call for a "political settlement" to the conflict.
Russian Foreign Minister Sergei Lavrov and his Arab counterparts on Saturday issued a five-point statement after holding talks in Cairo, calling for "unhindered humanitarian access" in Syria as well as an end to the violence.