Clashes between Syrian security forces and deserters killed 11 people in a village in Hama province Friday, while another eight died during a crackdown on protests in flashpoint Homs, rights activists said.
Thousands of protesters had taken to the streets on the Muslim weekly day of prayer that is a lightning rod in the six months of anti-regime protests in which the UN says 2,700 people have been killed by a fierce crackdown.
The protests were held under the slogan "victory for our Syria and our Yemen."
In Washington, meanwhile, Syrian Ambassador Imad Mustapha was called in to the State Department and "read the riot act" about an attempted attack Thursday on US Ambassador Robert Ford, spokeswoman Victoria Nuland told reporters.
A mob of nearly 100 Syrians chanting hostile slogans tried to storm an office in Damascus where Ford had arrived to meet opposition figure Hassan Abdelazim.
Mustapha "was reminded that Ambassador Ford is the personal representative of the president (Barack Obama) and an attack on Ford is an attack on the United States," Nuland said.
"He was also asked for compensation for our damaged vehicles," she added.
Ford spoke on his Facebook page Friday about the incident, saying the damage to the vehicles could not have been done "by eggs and tomatoes."
"Protesters threw concrete blocks at the windows and hit the cars with iron bars," he said.
The Assad regime had earlier accused Washington of inciting "armed groups" into violence against its army.
The mob attack came as the UN Security Council remained divided over whether to threaten Assad's regime with sanctions.
Britain, France, Germany and Portugal insist that any resolution must include at least the threat of sanctions against Assad, but Russia opposes any mention of sanctions.
On the ground, activists said those killed in Homs were shot dead by security forces who opened fire on protesters, while around 250 tanks and armoured vehicles entered Rastan, a major city in the province where there have been intense military operations against army defectors for days.
"Five civilians and six military and security agents have been killed today in the village of Kafar Zita during clashes between soldiers and agents on one side and deserters on the other," said the Syrian Observatory for Human Rights.
"Eight civilians were killed Friday in Homs province" by security force fire, added the British-based Observatory.
Much of the action centered on Rastan, about 180 kilometres (120 miles) from Damascus and a gateway to the north, where battles have been raging since Tuesday between the army and deserters, who refuse to fire on protesters.
The Observatory, citing a local activist, said an army officer was shot dead outside his home by pro-regime gunmen "for refusing to go to Rastan as ordered."
Rights activists also reported protests in a string of other towns and cities, including 10,000 people in Palmyre in central Syria, as well as thousands who poured out of mosques to stage demonstrations in Hama, also in the centre, Idlib in the northwest and Zabadani, just north of Damascus.
Security forces opened fire to disperse many of the protests, the activists said, without being able immediately to give an indication of other casualties.
The office of the UN High Commissioner for Human Rights in Geneva has said the death toll from the Syrian government's bloody crackdown has risen to more than 2,700 since March 15.