Syria's two main umbrella opposition groups, the Syrian National Council and the National Coordination Body for Democratic Change, had called for a boycott of the poll, describing it as being "stained with blood."

The referendum was also strongly criticized internationally. "The referendum in Syria is nothing more than a farce," German Foreign Minister Guido Westerwelle said. "Sham votes cannot be a contribution to a resolution of the crisis. Assad must finally end the violence and clear the way for a political transition."

US Secretary of State Hillary Clinton said there was "every possibility" that Syria would descend into civil war. She encouraged Syrians to rise up against the regime and for soldiers not to fire upon civilians.

Assad said that, if approved, the new constitution would lead to a multi-party parliamentary election within three months and end almost five decades of domination by his ruling Baath Party.

Russia, meanwhile, issued a strong warning to Western nations pushing for regime change in Syria, saying any military intervention without approval by the United Nations Security Council would not be tolerated.

Putin made clear Russia, which along with China blocked a Security Council resolution aimed at ending a government crackdown on opponents in Syria, would use its veto powers to block any US action.

Violence around Syria continued Sunday despite cues outside polling booths, with at least 31 people reported killed in fresh bloodshed.

According to the Syrian Observatory for Human Rights, nine civilians died as a result of Syrian army shelling in mainly opposition districts in the central city of Homs.

The International Committee of the Red Cross (ICRC) said it has resumed negotiations to allow the evacuation of Homs' artillery-battered Baba Amr district. Nearly 12 hours of talks on Saturday proved unsuccessful.

The annual session of the UN Human Rights Council was set to open on Monday with efforts to put more pressure on the Syrian regime on the agenda.

dfm, pfd/bk (Reuters, AP)