A bomb tore through a traffic-clogged district of downtown Damascus on Tuesday, killing at least 13 people and wounding scores, according to Syrian state television.

It was the second attack to strike at the heart of President Bashar al-Assad’s power base in as many days, underscoring his vulnerabilities as he battles a disparate but increasingly well-supplied rebel movement. Syrian Prime Minister Wael al-Halki narrowly escaped a car bombing that targeted his convoy Monday morning. At least nine people were reported killed in that blast. Opposition activists said the dead included at least two of Halki’s bodyguards and a driver in the convoy.

The attacks, which appear to have killed mostly civilians, also highlighted the growing ambiguities of a conflict that is in its third year and is affecting the resources, political agendas and even military strategies of other regional and international actors.

No group has asserted responsibility for the attacks. The Interior Ministry said the blast Tuesday in Marjeh, a commercial district of the capital, was merely “evidence that these terrorists and their masters are bankrupt.” State television said it was a “cowardly terrorist” act, echoing officials’ statements from the day before.

The Assad government has sought to portray the vast array of Syrian rebels — including opposition activists and Sunni Muslim radicals allied with al-Qaeda — as terrorists.

Mouaz al-Khatib, an opposition leader in exile, lashed out at Assad’s forces after the incident.

“Shelling civilians with Scud missiles and military aircraft, using chemical weapons and exploding cars among civilians and unarmed people are all cowardly actions,” he wrote. “Enough destroying Syria and killing its people. Enough.”

Arab Persian Gulf states, including Saudi Arabia and Qatar, have supplied arms to the rebels in recent months. And Iran, Russia and the Lebanese Shiite militant group Hezbollah have provided support to Assad’s forces.

In a televised speech Tuesday night, Hezbollah leader Hasan Nasrallah delivered a warning to Syrian rebels. “What you’re fighting is the Syrian army. So imagine what would happen if, in the future, other powers, other parties, other resistance forces decided to join in,” he said.

Hilal Khashan, a political scientist at the American University in Beirut, attributed the attacks to Assad’s enemies and said their ability to strike at the heart of what witnesses have described as an increasingly militarized capital — and to do it two days in a row — indicates that the government’s hold on power is eroding, albeit slowly.

Opposition forces have talked for months about a final battle for Syria’s predominantly Sunni capital, the nerve center of Assad’s rule. Assad is a member of the Shiite-affiliated Alawite sect.

The attacks this week do not suggest that the regime is about to collapse, Khashan said. But they send the message that the rebels are inside the capital, he added.

“These are psychological preparations ahead of a military operation. I think these attacks are aimed at gaining defectors and supporters,” he said.

At the same time, the indiscriminate nature of Tuesday’s attack also may lend credence to the government’s argument that its battle is with terrorists, rather than a force interested in political compromise — a contention that Assad has aggressively sought to sell to allies and foes alike.

Assad’s forces have shelled areas around Damascus heavily in recent weeks to counter rebel efforts to cut off access to the capital.

Television footage after the bombing Tuesday showed a chaotic landscape of burning cars in the city center, as firetrucks roared up amid the clatter of nearby gunfire. Lines of cars that appeared to have been waiting in traffic along a row of office buildings were battered by the blast, their windows shattered and their sides pushed in .

In one video compilation, posted on Facebook by a pro-government journalist, plainclothes agents and soldiers struggled to open damaged car doors to recover victims, while another group tried to lift a woman who was lying in a pool of blood on the ground. Two other men rushed to pick up a bloodied man who was splayed over his fallen bicycle, near a burning car.

Mazen, a Syrian who gave only his first name, said the blast shattered the glass of his office window, shards of which cut his arm as he sat at his desk. He said he ran into the street to find bodies and body parts littered.

“I saw a man wearing ordinary clothes carrying a machine gun and shooting in the air and screaming. He was far away, but I got scared, and I ran back into the building,” he said.