The latest escalations in different parts of Syria are another blow to international envoy Kofi Annan's peace plan, which aims to end the country's bloodletting. Annan brokered a cease-fire that went into effect on April 12 but has since been violated nearly every day since and never properly took hold.
The U.N. said several weeks ago that at least 9,000 people have been killed since the crisis began in March last year while Syrian activists say the violence has claimed the lives of more than 13,000 people.
The Damascus violence was a dramatic shift, since the capital has been relatively quiet compared with other Syrian cities throughout the uprising. Damascus and the northern city of Aleppo, the country's largest, are under the firm grip of Assad's security forces.
"Yesterday was a turning point in the conflict," said Maath al-Shami, an opposition activist in the capital. "There were clashes in Damascus that lasted hours. The battle is in Damascus now."
Blasts shook the neighborhoods of Qaboun and Barzeh until about 1:30 a.m. on Saturday.
"We spent a night of fear," one resident said, speaking on condition of anonymity for fear of reprisals. The resident said the shooting and explosions in the capital "were the worst so far."
As tanks fired shells, troops clashed with rebels in the two neighborhoods, al-Shami said via Skype. He said at least four people were killed.
The battles in the two neighborhoods began during the day Friday, when troops opened fire on anti-Assad protest marches, witnesses said. Also Friday, troops clashed with rebels from the Free Syrian Army in Damascus' Kfar Souseh district in fierce fighting sparked when the armed fighters attacked a military checkpoint in the area.
The FSA, which groups defectors from the Syrian military with protesters who have taken up weapons, had made an unusually public appearance Thursday night in Kfar Souseh, overtly joining a large opposition rally. The bolder moves were a strong sign the ragtag group is pushing to take its fight to the regime's base of power.
To the south, in Daraa, the Britain-based Syrian Observatory for Human Rights said 17 people were killed in the shelling, while the Local Coordination Committees said 19 civilians lost their lives. Both groups said dozens of people were also wounded in the shelling early Saturday.
Daraa is the city where the uprising against Assad's regime first erupted in March 2011. A Daraa-based activist, Adel al-Omari, said the shelling of the city's Mahata area began randomly and lasted until after midnight.
"People were taken by surprise while in their homes," said al-Omari, adding that regime targeted the neighborhood with mortars.
The LCC said that the dead included a father and his two children of the Abazeid family whose home was destroyed by the shelling. The group added that five of the dead were members of the Daloua family.
The LCC and the Observatory also reported shelling and clashes in the central city of Homs, one of the main battlegrounds of the uprising. Both groups said troops stormed Homs' posh neighborhood of Ghouta and the Observatory said security forces are conducting raids and searching for wanted people in the area.
In Turkey, Syria's main opposition council was scheduled later Saturday to elect a new leader, nearly three weeks after the resignation of its Paris-based president who earlier offered to step down over mounting criticism of his leadership. The executive committee of the Syrian National Council had asked Burhan Ghalioun to pursue his duties until a new president is elected.
SNC spokeswoman Basma Kodmani told Associated Press Television that the front-runner to replace Ghalioun would likely be Abdulbaset Sieda, a member of Syria's minority Kurd community.
The SNC has been plagued by infighting and divisions since its inception in September, complicating Western efforts to bolster the opposition. Ghalioun's resignation last month came just days after he was re-elected for another three months in a controversial vote in Rome.
On Friday, U.N. observers entered a farming helmet in the central province of Hama where activists said nearly 80 people were massacred on Wednesday. A U.N. spokeswoman said the observers could smell the stench of burned corpses and saw body parts scattered around the deserted village of Mazraat al-Qubair.
The observers were blocked by government troops and residents, and coming under small arms fire when they tried to enter the area on Thursday.
The scene held evidence of a "horrific crime," said U.N. spokeswoman Sausan Ghosheh.
The U.N. team was the first independent group to arrive in Mazraat al-Qubair, a village of about 160 people. Opposition activists and Syrian government officials blamed each other for the killings and differed about the number of dead.
Activists said that up to 78 people, including women and children, were shot, hacked and burned to death, saying pro-government militiamen known as "shabiha" were responsible. A government statement on the state-run news agency SANA said "an armed terrorist group" killed nine women and children before Hama authorities were called and killed the attackers.
Ghosheh, the U.N. observers spokeswoman, said the residents' accounts of the mass killing were "conflicting," and that they needed to cross check the names of the missing and dead with those supplied by nearby villagers. Mazraat al-Qubair itself was "empty of the local inhabitants," she said.
Associated Press writer Selcan Hacaoglu contributed to this report from Ankara, Turkey.