The detonation of a massive car bomb on Thursday near the heart of Damascus underscored a major shift that has brought sustained fighting close to the center of the capital for the first time during Syria’s two-year-old uprising.

It was unclear who was responsible for the attack, which killed at least 50 people and wounded more than 200 near the headquarters of Syrian President Bashar al-Assad’s Baath Party, activists and the official media said. But it marked one of the deadliest strikes yet in Damascus, where rebels have recently increased their pressure, with brazen daylight attacks on military checkpoints in the center of the city and what rebel fighters said were two mortar attacks aimed at Assad’s palaces.

After two weeks of clashes, some of the city’s hardest-hit neighborhoods have become desolate landscapes. Residents hunker down inside buildings with sporadic electricity and little food as the Syrian military hammers civilian areas with artillery and aerial raids, ordinary Syrians and opposition activists say. Snipers rule the streets in other neighborhoods, taking potshots at anyone who crosses their path.

Rebels have launched some striking attacks in the capital before: four top security officials, including Assad’s brother-in-law, were killed in a bomb attack in July. But only recently have they demonstrated an ability to attack and hold territory even closer to the heart of Damascus.

Still, neither side appears to have built up decisive momentum. Instead, both sides seem to be digging in for a long showdown, a new and bloody phase of the conflict that could lead to the destruction of vast swaths of Damascus, devastation similar to what fighting has done in Aleppo, Syria’s largest city and its commercial center.

“The fight for Damascus is far from over,” said Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy. “Does it mean the regime is in good shape? No. But I don’t think a rapid collapse is going to happen.”

Rebels had fought mostly on the periphery of the capital for months, including heavy clashes in December near the international airport, more than five miles from the city center. But this month’s clashes have been concentrated primarily in the eastern and southern suburbs of Damascus, Sunni-dominated areas that have long supported the opposition. And the deadly bombing of the Baath Party building in the city center showed that the violence is spreading beyond those areas and into the heart of the capital.

Also on Thursday, two car bombs targeted a security facility in the Barzeh neighborhood, killing at least 10 soldiers and three civilians, according to opposition groups. No rebel groups asserted responsibility for those attacks.

The Syrian government and rebel forces accused each other of carrying out the attack on the party headquarters, which killed mostly civilians. But the Syrian government has often denied reports of rebel advances in the capital, saying such claims were “intended to lift the morale of the terrorists.”

The recent violence hasn’t taken place only around government or military targets. At least two mortar rounds hammered a complex next to a sports stadium in central Damascus on Wednesday, killing a soccer player from a team based in Homs, according to state media. The 19-year-old player, Youssef Suleiman, had been preparing for a match when the rounds struck.

The intensified fighting in Damascus and other cities around the country has taken a noticeable toll. About 10,000 people have died since the beginning of the year, bringing the total deaths from the war to a staggering 70,000, according to estimates by the United Nations. And about 860,000 Syrians have fled to neighboring countries, the United Nations has said, without about one-quarter of them escaping in the past two months.

“The situation in Syria is getting worse. The violence is causing widespread destruction and having a devastating impact on the lives of ordinary Syrian women, men and children,” U.N. humanitarian chief Valerie Amos said Tuesday at a meeting in Geneva on the humanitarian crisis in Syria.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

Last month, during Amos’s most recent visit to Damascus, the devastation from the conflict was unavoidable. “I was conscious of the constant shelling, and I have seen firsthand the destruction of lives, of infrastructure and the erosion of basic social services like health and education,” she said at the meeting.

One of the most dangerous activities, Damascus residents say, is a simple trip to buy bread, a routine activity now turned deadly. And in many areas, the price of bread has increased fivefold to more than a dollar apiece, a large sum for the many families that are barely making ends meet.

The Syrian government has severely limited journalists’ access to the capital; accounts of life there in this report came from interviews with a dozen activists, rebel fighters and Damascus residents, including some who have recently fled the city and are now in Lebanon.

Away from the front lines, the capital has been gripped by paranoia and fear as the Syrian military and the pro-government militia known as shabiha have set up many new checkpoints and rolled out heavy weapons in new positions to defend the city, according to residents and opposition activists reached by phone and online.

“The situation is extremely tense in the heart of the capital,” said an activist in Damascus who goes by Hamza al-Dimashqi. “The city of Damascus is being strangled even more now.”

Coordinated assault

The push into the capital came after weeks of stalemate in the area surrounding the city, where rebel forces were held off by the much better armed and equipped Syrian military. Each time individual rebel units mounted an assault, government forces struck back hard with aerial bombs, helicopter gunships and infantry troops.

But rebel forces managed to chip away at the military’s defenses and have now launched a coordinated assault with several units from the Damascus area and other parts of the country, including some hard-core Islamists, rebel fighters and commanders say.

“The areas around Damascus have been liberated, and now we are moving into the city,” said a unit commander from the Damascus countryside who uses the nom de guerre Obeida Arbeeni and was reached by Skype. “We are now fighting better because we have captured many weapons.” Rebel fighters say they have boosted their armory with heavier machine guns and more ammunition, and they boast of using a confiscated tank in one recent attack on a checkpoint in eastern Damascus.

Still, the rebels could face serious problems taking the fight from the suburbs to densely populated areas of the capital, particularly if they want to win over residents in new areas.

“Our biggest challenge is the civilians, especially since the regime is trying to turn them against us,” said the spokesman for the rebels’ Damascus military council who uses the nom de guerre Abu Qutada.

Some of the heaviest recent fighting has been in Jobar, a neighborhood in the northeastern part of the capital only a little more than a mile from walls of the historic old city. Most of the residents have cleared out completely, activists say, and many of the pharmacies and bakeries have shut down, making life even more difficult for those who remain.

“The shelling is falling randomly on the streets so you might be hit by one while walking,” said a pro-opposition journalist in Jobar who goes by Abu Hassan al Dimashqi.

Zamalka, just east of Jobar, has been hit equally hard. Bakeries and gas stations have even been attacked by the military, opposition activists say.

“The regime is firing all kinds of weapons at us,” said an opposition activist in Zamalka who goes by Abu Malik. “The regime has no red lines that it has not crossed,” he added, mentioning the use of MiG jets and mortars.

Social breakdown

The threat to civilians hasn’t come from combat alone. An atmosphere of lawlessness has descended on some neighborhoods, and the capital is rife with stories of kidnapping for ransom, robbery and rape. Residents blame the combatants as well as criminal gangs that don’t take sides in the civil war.

In some areas of Damascus, opposition fighters have eased suffering by handing out bread and fuel and fixing electrical lines, according to residents and opposition activists.

But the conflict appears to be creating a shift in the sectarian makeup of neighborhoods in Damascus, residents say. Many Alawites, the Shiite sect of Assad and top government figures, fear revenge attacks from the opposition, which is dominated by Sunni Muslims, and some have fled rebel-controlled areas to other parts of the capital or even places outside the city.

Although the trend seems in its early phases, and the evidence is largely anecdotal, the hardening of sectarian lines could lead to a deep and intractable conflict, not unlike what happened in Baghdad because of sectarian violence there, analysts say.

“The country is in a situation where it’s going to be a de facto partitioned state going forward for some time,” Tabler said.

Suzan Haidamous contributed to this report.