Hama deaths prompt call for U.N. action NEW: At least 20 are killed in violence Thursday, the opposition says The Arab League's ministers gather in Cairo to discuss Syria The Syrian opposition demands the United Nations hold an emergency session The state-run news says terrorists in Aleppo killed a school teacher (CNN) -- A military rocket attack killed more than 70 people in the Syrian flashpoint city of Hama, in what activists said Thursday is one of the deadliest incidents in the grinding conflict. The incident prompted the Syrian opposition on Thursday to call on the United Nations Security Council to hold an emergency session to take up the issue of protecting civilians. The Wednesday strike, in the Masha'a Altayar neighborhood, caused many poorly constructed buildings to collapse. Video showed people milling around the rubble. One activist said more than a dozen children were pulled from the wreckage. Syrian President Bashar Al-Assad's government blamed terrorist groups for the Hama deaths. The state-run Syrian Arab News Agency said a terrorist group was building a bomb that exploded and killed 16 people, including children. On Thursday, security forces fired on a demonstration that coincided with a general strike to mourn the dead, said the opposition Local Coordination Committees of Syria (LCC). The demand by the opposition Syrian National Council comes the same day the Arab League called an emergency meeting in Cairo in which ministers will take up the issue of Syria's fragile truce, which showed little sign of taking hold amid reports of violence in several cities. "The regime is committing all sorts of violations to Annan's plan and until this moment it has not abided by any of the plan's points," the council said in a statement denouncing the Hama incident. Kofi Annan presented the six-point plan to end the hostilities in his role as the U.N. point man on the Syrian crisis. The national council said in a written statement that the "international community, represented by the United Nations and its Security Council, bears responsibility for what is happening now on the ground." "We call for an emergency session (at the United Nations) to issue a resolution to protect Syrian civilians," the statement said. The council said it will continue to support the Free Syrian Army, the anti-regime fighter force, to protect "unarmed people," regardless of the future of Annan's plan. But hope for the plan's success is wavering amid opposition claims that al-Assad's security forces continue to use snipers and tanks to attack rebels in volatile cities such as Hama. "This is among the deadliest attacks, and is further proof that the Assad regime has no intention of implementing the Annan plan," LCC spokeswoman Rafif Jouejati said, referring to the Wednesday strike in Hama. The observers will also monitor the implementation of the peace plan, which calls for the government and the opposition to end the bloodshed, provide access to the population for humanitarian groups, release detainees and start a political dialogue. By Monday, 30 observers are expected to be in the country, with that number swelling to 100 by month's end. At least 20 people were killed across the country on Thursday, the LCC said. Two died in a in shelling near Deir Ezzor in eastern Syria that also wounded 25, according to the opposition. Terrorists set off a car bomb that killed a school teacher in the city of Aleppo Thursday, the state-run Syrian Arab News Agency said. The report said the attackers targeted "national expertise." Syrian authorities say terrorists have been targeting educators, engineers, and medical personnel during the crisis. The Syrian Observatory for Human Rights reported the killing of a man in a car bombing, but didn't identify him. The Syrian National Council condemned the international community for continuing to give al-Assad's government time to implement the peace place because it gives "the criminal regime more time to kill." Al-Assad has previously reneged on pledges to end the violence. "It's like all the other lies before, and an excuse for the government to regroup and hunt down the wanted," said Capt. Ismail, a rebel commander. The United Nations estimates at least 9,000 people have since died in the conflict, while activist groups put the death toll at more than 11,000. CNN's Mohamed Fadel Fahmy, Arwa Damon, Amir Ahmed and Joe Sterling contributed to this report.