Aleppo, Syria's second largest city and its commercial hub, was also the target of car bombings on February 10 that killed 28 people.
On Saturday, two car bombs killed 27 people and wounded 140 others in the heart of Damascus, mostly civilians, the interior ministry said.
It blamed "terrorists" for the attacks near police and air force headquarters.
Damascus and Aleppo are both seen as having high levels of support for President Bashar al-Assad.
"Yesterday's explosions were carried out by terrorists supported by foreign powers which finance and arm them," said Al-Baath newspaper of the Damascus bombings.
The newspaper is the mouthpiece of Assad's ruling party of the same name.
"The two attacks... aim to disrupt Annan's mission and to foil international efforts to find a political solution to the crisis," it said, referring to UN-Arab League peace envoy Kofi Annan.
Ath-Thawra, another official daily, accused Qatar and Saudi Arabia, which have called for rebels fighting the Assad regime to be armed.
But the opposition Syrian National Council accused the regime of staging the car bombings.
Damascus was trying to terrorise its own citizens and make it look as if the country was under threat from Al-Qaeda, they said, calling for an international commission of inquiry.
Technical experts from the UN and OIC were meanwhile working in Syria to assess the humanitarian impact of the regime's deadly crackdown on protests since March 2011.
"The joint OIC-UN mission entered Syria on Friday to carry out an evaluation of humanitarian aid," on a mission led by the Syrian government, OIC assistant secretary general Atta al-Mannan Bakhit told AFP.
The mission, with three OIC experts in the team, would cover 15 cities, he added. It would submit a report to the Saudi-based Islamic grouping and the United Nations on the humanitarian needs of the Syrian population.
UN humanitarian chief Valerie Amos, who held talks in Damascus earlier this month, has said the experts would join the assessment mission to Daraa, Homs, Hama, Tartus, Latakia, Aleppo, Deir Ezzor and rural zones around Damascus.
UN experts and staff of the 57-member OIC would "accompany the mission and take the opportunity to gather information on the overall humanitarian situation and observe first-hand the conditions in various towns and cities," she said.
Amos's spokeswoman Amanda Pitt told AFP the government-led mission set off on Sunday morning "and proceeded to Homs."
The UN and OIC staff were accompanying the mission "to get a better understanding of the situation and needs in areas round the country and gather information," she said.
The technical experts would stay in Syria for seven to 10 days, Pitt added.
The United Nations estimates that the past 12 months of violence have forced more than 30,000 Syrians to flee to neighbouring states, while another 200,000 have been displaced within the country.
Activists say the year-long conflict has cost more than 9,100 lives.
In parallel with the mission Jakob Kellenberger, president of the International Committee of the Red Cross, has flown to Moscow for talks Monday with Russian Foreign Minister Sergei Lavrov on the "extremely difficult" humanitarian situation in Syria's protest centres.
"A daily ceasefire of at least two hours is imperative to allow the evacuation of the wounded," he said ahead of the mission to Moscow, an ally of Damascus which is seen as having some influence on Syria's leadership.
Former UN chief Annan, who met Assad in Damascus last weekend, has also ordered a team of experts to Syria to discuss a possible ceasefire and an international monitoring mission, his spokesman said.
His team will head to Damascus from New York and Geneva on Monday.
Inside Syria itself, in addition to those killed in the Aleppo car bombing, another 26 people were killed nationwide, said the Syrian Observatory, a Britain-based monitoring group.
Those killed were 19 civilians, six soldiers and one army deserter.
It also reported that security forces beat up and detained opposition figure Mohammed Sayyed Rassas, a leader of the National Coordination Committee for Democratic Change, and several youths at a Damascus protest.