DAMASCUS, Syria — A car bomb near the Damascus headquarters of Syria's ruling party killed 35 people on Thursday, while a government airstrike on a rebel field hospital in southern Daraa left 18 dead, opposition activists and state media reported.
The Damascus car bomb was one of at least three attacks in the heart of the city. A second blast shook another neighborhood and mortar rounds exploded near the Syrian Army General Command.
It was the third day of attacks on the center of Damascus.
For months, rebels have been trying to bring their fight to topple President Bashar Assad into the center of the capital, but have managed little more than brief incursions and frequent skirmishes in outlying neighborhoods.
The latest bombings and the recent mortar attacks suggest they may be shifting to guerrilla tactics to destabilize the seat of Assad's power.
The most deadly attack struck a main street on the edge of the capital's central Mazraa neighborhood, near the headquarters of Assad's Baath party and the Russian Embassy, as well as a mosque, a hospital and a school.
TV footage of the blast site showed firemen dousing a flaming car with hoses and lifeless and dismembered bodies blown into the grass of a nearby park.
Witnesses at the scene said a car had exploded at a security checkpoint between the Russian Embassy and the central headquarters of the ruling party.
"It was huge. Everything in the shop turned upside down," one local resident said. He said three of his employees were injured by flying glass that killed a young girl who was walking by when the blast hit.
"I pulled her inside the shop but she was almost gone. We couldn't save her. She was hit in the stomach and head," he said, speaking on condition of anonymity for fear of retribution for speaking with foreign media.
Ambulances rushed to the scene of the blast, which shattered windows and sent up a huge cloud of smoke visible throughout much of the city, witnesses said.
The Britain-based activist group the Syrian Observatory for Human Rights said at least 42 people were killed, most of them civilians. Some members of the Syrian security forces were also killed, it said.
Syrian state TV called it a "terrorist" attack by a suicide bomber. It said at least 35 people were killed and more than 200 wounded. The state news agency published photos of two dead bodies lying in the street.
There was no way to immediately reconcile the differing death tolls.
Russia's state owned RIA Novosti news agency quoted a Russian Embassy official as saying the Embassy building had been damaged in the blast but no one was hurt.
In a separate attack, Syrian state TV said mortar shells exploded near the Syrian Army General Command in central Damascus, causing no casualties. The station said the building was empty because it was under renovation.
The Britain-based activist group Syrian Observatory for Human Rights said two mortar rounds struck near the building but did not report casualties.
On Wednesday, two mortar shells exploded next to a soccer stadium in Damascus, killing one player. The day before, two mortar shells blew up near one of Assad's three palaces in the city, causing only material damage.
Between the car bomb and the mortar attack near the army command, a security official reported another blast in the capital's northeastern Barzeh neighborhood. He had no other information and spoke on condition of anonymity because he was not authorized to brief the media.
The Observatory, which relies on a network of anti-regime activists inside Syria, said two car bombs had exploded near security centers in Barzeh, followed by intense clashes between rebels and security forces.
Syrian state media also reported that security forces in Damascus had arrested a second, would-be suicide bomber driving a car full of explosives near the site of the Mazraa bombing.
Damascus has so far mostly avoided the large-scale violence that has destroyed other Syrian cities, though deadly car bombings have targeted government buildings in the capital.
In May 2012, twin car bombs exploded outside a military intelligence building, killing 55 people in the deadliest attack against a regime target in the capital since the uprising began 23 months ago.
And in July, rebels detonated explosives inside a high-level crisis meeting in Damascus that killed four top regime officials, including Assad's brother-in-law and the defense minister.
Following that attack, rebel groups who have established footholds in suburbs of the capital pushed in, clashing with government forces for over a week before being routed and pushed out.
Since then, government jets have heavily bombed rebel-held suburbs and rebels have managed only small incursions on the city's south and east sides.
In Daraa, where Syria's uprising began nearly two years ago, the Observatory said the 18 people killed in the airstrike included eight rebel fighters, three medics, one woman and one young girl.
A video posted online said to be of the event showed the bodies of dead and wounded people being loaded in to the back of trucks and moved to another location. Some were bloody and had bandaged heads, while others were carried out on stretchers.
Syria's conflict began in March 2011 with political protests against the government and has since evolved into a civil war between Assad's regime and hundreds of rebel groups seeking to topple it. The U.N. says some 70,000 people have been killed in the conflict so far.
International diplomacy has failed to slow the fighting.
On Thursday, British Foreign Secretary William Hague said his message to Assad is that "it is time to go."
He said the senseless killing must be brought to an end through a credible political process leading to a transition in Syria.
He also called on Assad to respond to a dialogue offer made recently by Syrian opposition chief Mouaz al-Khatib.
"A political settlement, a political agreement on a transition is the way forward in Syria to bring to an end this terrible and unacceptable loss of life."
Al-Khatib has said he is open to talks with the regime that could pave the way for Assad's departure, but that the Syrian leader must first release tens of thousands of detainees. The government has refused.
Hubbard reported from Beirut. Associated Press writer Zeina Karam contributed reporting from Beirut.