At least 52 people died in clashes and bombing across Syria on Thursday, with activists calling for another day of protests as the uprising against President Bashar al-Assad enters its 16th month.
Fourteen people were also wounded when a suicide bomber blew up a vehicle near an important Shiite Muslim shrine in the capital.
Another car bomb in the northwestern city of Idlib killed and wounding a number of soldiers, the Syrian Observatory for Human Rights said.
Monitors say more than 14,100 people, mostly civilians, have died since a peaceful uprising erupted on March 15, 2011, prompting a bloody crackdown by Assad's forces that eventually prompted an armed reaction.
More than 2,302 have reportedly died in the past month alone.
Official news agency SANA said a vehicle in Damascus exploded in a garage 50 metres (yards) from the shrine and caused substantial damage, with the "terrorist" who launched the operation killed.
The Observatory, citing anti-regime activists, said it went off near security offices, damaging the apparent target as well as the shrine, as seen in a video posted on the Internet.
Most of Syria's 22-million population are Sunni Muslims, while its minorities include Alawites, an offshoot Shiite community to which Assad belongs.
Hundreds of thousands of pilgrims, mainly from Syria's ally Iran, travel each year to the shrine of Sayyida Zeinab, a granddaughter of the Muslim Prophet Mohammed, in an area of south Damascus where many Iraqi refugees live.
As the death toll soars, Amnesty International accused the Syrian regime of committing crimes against humanity to punish communities supporting rebels.
The London-based group called for an international response after saying it had fresh evidence that victims, including children, had been dragged from their homes and shot dead by soldiers, who in some cases then set the bodies on fire.
"This disturbing new evidence of an organised pattern of grave abuses highlights the pressing need for decisive international action," said Amnesty's Donatella Rovera on the release of the 70-page report entitled Deadly Reprisals.
The group interviewed people in 23 towns and villages and concluded that government forces and militias were guilty of "grave human rights violations and serious violations of international humanitarian law amounting to crimes against humanity and war crimes."
As on nearly every Friday since the uprising began, activists have called for nationwide demonstrations after weekly Muslim prayers, with this week's slogan being "Always prepared for a strong mobilisation."
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Areas in the provinces of Homs, Daraa, Damascus, Aleppo, Deir Ezzor and Idlib were all targeted, the Observatory said.
Meanwhile, UN observers visited Al-Haffe town in the Mediterranean province of Latakia, a day after Syrian authorities said the area had been "cleansed" of rebel fighters, a UN spokeswoman in Damascus said.
On Wednesday, rebels withdrew from the besieged town and nearby villages that had been under intense regime shelling for eight days, the Observatory said.
The UN Supervisory Mission in Syria (UNSMIS) said the observers reported finding it all but deserted with a strong stench of dead bodies and most state buildings gutted.
"Most government institutions, including the post office, were set on fire from inside," a statement said. "Archives were burnt, stores were looted and set on fire, residential homes appeared rummaged and the doors were open."
It added that "a strong stench of dead bodies was in the air and there appeared to be pockets in the town were fighting is still ongoing."
It said the number of casualties was still unclear.
State television said the observers had "inspected the vandalism and destruction wrought by the terrorists."
The United Nations and opposition activists had expressed fears of a massacre if pro-government forces entered the town, just 16 kilometres (10 miles) from Assad's mainly Alawite hometown of Qardaha.
Opposition sources said anti-Assad groups are to meet in Istanbul on Friday and Saturday in a bid to settle their differences and close ranks.
Senior members of the main opposition Syrian National Council, the Kurdish National Council and smaller groups are to take part, they said.
"It is kind of a last call to join us," an SNC source said on condition of anonymity.
On the diplomatic front, British Foreign Minister William Hague urged Russia and Iran to use their "full influence" over ally Syria to achieve a peaceful end to the bloody uprising.
China, meanwhile, said it disapproved of "one-sided" sanctions and pressure on Syria after France raised the prospect of a new raft of punitive measures against Assad's regime.
In other developments, UNSMIS said observers monitored the release of 239 detainees in Damascus, and another 13 in Idlib and Deir Ezzor.
It said the government had told it a total of 500 are being freed nationwide.