At least 90 people have been killed in an attack by Syrian government forces and loyalists on Houla, a town in Homs province, activists have said.
The victims of Friday's assault included at least 25 children, killed after government forces tried to break into the town, the UK-based Syrian Observatory for Human Rights said.
Houla has been the scene of frequent anti-government protests since the uprising against Syrian President Bashar al-Assad began in March last year. The town has also become a hub for opposition fighters.
Syria's main opposition bloc put the toll at more than 100 and urged the UN Security Council to convene an emergency meeting to examine the massacre.
"More than 110 people were killed [half of whom are children] by the Syrian regime's forces", the Syrian National Council said in a statement.
"Some of the victims were hit by heavy artillery while others, entire families, were massacred," said the statement by Bassma Kodmani, the council's head of foreign relations.
A team of UN observers visited the Homs area to assess the situation on Saturday. Some activists complained, however, that they just visited the village of Taldou, at the edge of Houla, rather than entering the town.
Videos 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.
Amer al-Sadeq of the Syrian Revolution Co-ordinators Union blamed the killings on Assad loyalists from surrounding villages who he said "have been armed systematically by the regime since the beginning of the uprising".
"In the face of horror, the international community must mobilise still further to stop the martyrdom of the Syrian people. "
- Laurent Fabius, French foreign minister
The Local Co-ordination Committees activist network said government forces had shelled the town before "armed militias slaughtered entire families in cold blood".
Activists told Al Jazeera that the town came under shelling again on Saturday.
Hadi al-Abdallah, speaking from Homs, said Houla is under the control of the Free Syrian Army, which means government troops cannot enter the town. Instead, they are launching shells from a distance in a bid to defeat the rebels.
Syrians turned out in several locations around the country on Saturday to protest against the killings.
In Kafr Nabel, in the northern province of Idlib, scores took to the streets to honour the dead, chanting: "We sacrifice our soul and life for you, O people of Houla," according to a video posted on YouTube.
In a Damascus neighbourhood, women filmed hiding their faces were carrying papers that read: "The Syrian regime kills us under supervision of the UN observers" and 'Banish the UN tourists'.
French Foreign Minister Laurent Fabius condemned the assault and called for greater international action.
"In the face of horror, the international community must mobilise still further to stop the martyrdom of the Syrian people," he said in a statement.
"I am making immediate arrangements for a Friends of Syria group meeting in Paris."
The US, France, Britain, Germany, Saudi Arabia and Qatar are leading members of the group, which has held several meetings calling for tougher action against the Assad government.
The latest flare-up of violence came as Ban Ki-moon, the United Nations secretary-general, blamed the government for much of the "unacceptable levels of violence and abuses" occurring every day in the 14-month-long crisis in Syria.
In a report to the UN Security Council on Friday, Ban cited the government's continued use of heavy weapons, reports of shelling and "a stepped-up security crackdown by the authorities that has led to massive violations of human rights by government forces and pro-government militias".
In-depth coverage of escalating violence across Syria
Ban said there had only been "small progress" on implementing the six-point joint UN-Arab League plan brokered by international envoy Kofi Annan.
The UN chief called on the government to keep its pledge to immediately stop the violence, pull heavy weapons and troops out of populated areas, allow humanitarian workers to help civilians in need and end human rights abuses.
Ban also called on all elements of the opposition to stop the violence and respect human rights.
The secretary-general said 271 of 300 unarmed UN military observers authorised by the council to help end the conflict were on the ground and appeared to have had a "calming effect" in key cities.