President Bashar Assad's forces killed at least 90 civilians, including 13 children, in central Syria, activists said.
It is one of the highest death tolls in one specific area since an internationally-brokered ceasefire went into effect last month.
Syrian troops using tanks, mortars and heavy machine guns pounded the area of Houla, a region made up of several towns and villages in the province of Homs, yesterday, British-based Syrian Observatory for Human Rights and the Local Co-ordination Committees activist groups said.
Both groups said at least 50 people were killed. The observatory, which has a network of activists around the country, said the dead included 13 children. It added that about 100 people were wounded.
An amateur video posted online by activists showed more than a dozen bodies lined up inside a room. They included about 10 children who were covered with sheets that only showed their bloodied faces.
"Houla was subjected to a massacre," a man could be heard saying inside the room.
The observatory said in one incident in Houla, a family of six was killed when their home received a direct hit.
Homs has been among the hardest hit provinces in a government crackdown since the uprising against Assad's regime began in March last year. The United Nations said several weeks ago that 9,000 people had been killed in Syria in the past 15 months. Hundreds more have died since.
Attacks like yesterday's, as well as strikes by rebel forces on government troops, have persisted despite the deployment of more than 250 UN observers who have fanned out across Syria to monitor a ceasefire brokered by international envoy Kofi Annan.
Despite the daily violations, UN secretary general Ban Ki-moon said that there was no "plan B" for the Annan initiative.
The northern city of Aleppo, a major economic hub, has remained largely supportive of Assad throughout the uprising but anti-regime sentiment has been on the rise in recent weeks.
Yesterday, Syrian forces fired tear gas and live ammunition to disperse thousands of protesters in Aleppo calling for Assad to be ousted, killing five people, activists said.
Aleppo-based activist Mohammad Saeed said more than 10,000 people were protesting in the city.
"The regime is desperately trying to put down the protests in Aleppo but all this violence will backfire," he said. He added that security forces shot dead five people, including a 12-year-old boy identified as Amir Barakat.
"Wounded and bloodied people are in the streets," Mr Saeed said.
Meanwhile a group of Lebanese Shiites who were kidnapped in Syria were freed, three days after gunmen abducted the men as they returned from a religious pilgrimage.
The kidnappings fuelled fears that Lebanon is getting drawn into the bloody conflict in neighbouring Syria.
In the hours after Tuesday's abductions, protests erupted in Beirut's Shiite-dominated southern suburbs, where residents burned tyres and blocked roads.
Lebanese prime minister Najib Mikati confirmed the men were released, saying they were "in good health and on their way to Beirut".
The pilgrims were believed to have been returning from a trip to visit holy sites in Iran when they were abducted.
The hostages were believed to be 11 Lebanese and one Syrian driver. Lebanese and Syrian officials have blamed Syrian rebels for the kidnappings, but nobody has claimed responsibility.
Sunnis form the backbone of the Syrian revolt, which has unleashed seething sectarian tensions. Assad and the country's ruling elite belong to the tiny Alawite sect, which is an offshoot of Shiism.
The leader of Lebanon's powerful Shiite militant group Hezbollah, which has stood by the Syrian regime, welcomed the pilgrims' release. Speaking by satellite link, Sheik Hassan Nasrallah said the group's support for Syria is firm.
"If you aim to put pressure on our political stance, this will not make any difference," he said of the kidnappings.
The abductions came at a time of deep tension in Lebanon over Syria. The countries share a web of political and sectarian ties and rivalries, which can quickly turn violent. Clashes linked to the Syria conflict have killed at least 10 people in Lebanon the past two weeks.
Nasrallah's comments appeared to be an attempt to de-escalate the recent tensions.
"I also thank all the people who controlled their emotions and responded to our call for calm, wisdom and patience," he said, referring to a speech he gave earlier this week calling on his supporters not to take to the streets in anger.