Syria says it has released more than 500 people imprisoned for their role in anti-government protests, but an opposition group accused the regime of torturing and killing an even larger number of jailed people since uprisings began in March.

Syrian state television said Thursday that those released had not taken part in what it called the killing of Syrians – a reference to the protest movement against the government of President Bashar al-Assad.

At the same time, however, the activist group Avaaz accused the Assad government of murderous tactics and torture: specifically, the killing of 617 people in overcrowded jails and illegal detention centers since the Syrian government began its crackdown on protests.

The Avaaz group says its count shows nearly 7,000 people have been killed in the Syrian uprising, compared to a U.N.-compiled toll of 5,000 dead. It challenged Arab League investigators to expose the regime's “torture chambers” and bring an end to harsh treatment of protesters.

Separately, other activist groups said at least four people were killed Thursday during clashes between protesters and Syrian forces in the eastern Deir Ezzor region.

Avaaz, a global activist group that operates primarily online , says it is dedicated to bringing “people-powered” movements to prominence and to encourage democratic governments. It contends it verified the fate of every victim on its list through multiple independent sources.

Earlier this week, Arab League Secretary-General Nabil El Araby said the League's team in Syria had secured the released of about 3,500 prisoners. However, the head of the British-based Syrian Observatory for Human Rights said his contacts have not seen any detainees released.

Opposition groups in Syria have sharply criticized the Arab League observer mission, saying its inaction has allowed Mr. Assad to continue his forces' crackdown against protest.

On Thursday, Qatar's prime minister conceded the Arab League mission has made some mistakes in Syria. The league's ministers are due to meet Saturday in Cairo to review the project.