A suicide bomber on Friday killed 26 people and wounded dozens in the Syrian capital with the government vowing an "iron fist" response.
The bomber blew himself up near a school in in the historic Midan quarter, Interior Minister Lieutenant General Mohammad al-Shaar was quoted as saying by the SANA news agency. Another 17 civilians were shot dead in the now daily unrest, activists said.
Authorities found the complete and body parts from 26 people while 63 others were wounded in the second blast in two weeks in the heart of Damascus, the minister added.
The interior ministry branded the bombing "a new terrorist escalation" and said more than 10 kilos (22 pounds) of explosives was used.
"We will strike with an iron fist all those who undermine the security of the nation," the ministry said in a statement.
Syrian opposition factions blamed President Bashar al-Assad's government for the bombing and called on Arab and Western forces to end the bloodshed.
"We hold the regime, its agents and its gangs, fully responsible for this crime," said the Muslim Brotherhood, which had also accused the government of orchestrating twin suicide attacks in Damascus on December 23.
The Syrian National Council said Friday's bombing "clearly bears the regime's fingerprints." The SNC is an umbrella group which includes the Muslim Brotherhood.
The United States and United Nations condemned the bombing.
"We categorically condemn this attack," State Department spokeswoman Victoria Nuland said, adding that the solution to the Syrian crisis is "for Assad to step aside and for a national dialogue to begin."
"I remain gravely concerned at the deteriorating situation in Syria, where thousands have lost their lives since March last year and people continue to be killed each day," said UN leader Ban Ki-moon.
"I say again that all violence is unacceptable and must stop immediately," he added.
Syrian state media showed gruesome images of rescuers gathering body parts in plastic bags.
The December 23 bombs killed 44 people. The authorities blamed Al-Qaeda while the opposition accused the government.
Friday saw similar counter-accusations.
The ruling Baath party called the attacks "a terrorist act that is a part of the plot hatched against Syria," state media reported.
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The plot, it said, "coincides with the statements made by opposition groups and by French and American officials."
The Muslim Brotherhood called for an international probe, claiming the attack benefited the regime.
"The killings in Syria will continue and the Syrian regime will keep hiding behind Al-Qaeda and the terrorists... unless someone confronts the regime and takes it to account for its crimes," said the Brotherhood.
The SNC said the UN Security Council had to take up the bloodshed, which UN estimates in December said had killed more than 5,000 people since pro-democracy protests erupted in March.
"The SNC believes that an agreement on a joint effort between the Arab League and the United Nations Security Council represents a first step toward the urgent and necessary measures to assure the protection of civilians, and to ensure that the regime does not commit additional bombings and killings," it said.
Activists have called for an Arab League observer mission to admit its failure to stem nearly 10 months of bloodshed and hand over to the United Nations.
UN leader Ban reaffirmed an offer to train the Arab League observers.
The Arab mission has been in Syria since December 26 trying to assess whether the government is complying with a peace accord aimed at ending its deadly crackdown.
But there has been no let-up in the violence and new criticism of the observers.
Seventeen civilians were shot dead on Friday, including eight around Damascus and six in the central city of Hama, the Syrian Observatory for Human Rights told AFP. Three army deserters also died, it said.
Protests were reported in Aleppo and Homs, the port of Latakia, and Douma on Damascus's northern outskirts.
Two explosions shook the eastern oil hub of Deir Ezzor, wounding five security personnel, SANA said.
The Arab League is to meet on Sunday in Cairo to discuss the observer mission, whose credibility continues to be questioned.
"We support the Arab League which has sent observers to Syria but this mission is not at present able to do its job properly," France's Foreign Minister Alain Juppe said.
The head of the rebel Free Syrian Army has called on the Arab League to admit failure, and urged the bloc to seek UN help.
"We call on the Arab League to step aside and let the United Nations take over responsibility as it is more apt to find solutions," Colonel Riyadh al-Asaad told AFP in Beirut.
Qatari Prime Minister Sheikh Hamad bin Jassem al-Thani, who heads an Arab League task force on Syria, has admitted "mistakes" in the monitoring mission.