At least 31 Syrian civilians and soldiers were killed in widespread clashes Sunday, as voters cast ballots for a new draft constitution amid opposition calls for a boycott of the poll. The proposed document would end the ruling Baath Party's decades-long rule.

The Britain-based Syrian Observatory for Human Rights said the continued military bombardment of opposition districts in the protest hub of Homs left nine civilians dead Sunday, while rebel fighters killed four soldiers in the city.

The group said eight civilians and 10 security force personnel were killed in violence elsewhere in Syria as polling was underway.

In areas like Homs, where government shelling has left hundreds dead, or the rebel strongholds of Idlib in the northwest and the southern region of Daraa, turnout is likely to be minimal. Activists in Homs said no voting appeared to be taking place, and Internet video showed some dropping ballots into the trash.

Despite the violence in provincial cities across Syria, voting on the constitution went ahead in calmer areas. In the capital Damascus, where President Bashar al-Assad retains support among religious minorities and the business class, many said they were eager to vote.

If approved, the referendum it would drop an article making the ruling Baath party the leader of state and society, allow political pluralism and enact a presidential limit of two seven-year terms. But the limit would not be enforced retrospectively, meaning that Mr. Assad, already in power for 11 years, could serve another two terms after his current one expires in 2014.

Critics have denounced the vote as a cruel joke, saying Mr. Assad must relinquish power after seeking to violently crush an 11-month uprising against his autocratic rule.

The two main umbrella opposition groups – the Syrian National Council and the National Coordination Body for Democratic Change in Syria – called for a boycott.

A spokesman for the International Committee of the Red Cross said talks with Syria's government and opposition groups produced “no concrete results” Saturday. The ICRC said negotiations with both sides will continue Sunday in an attempt to enter Homs and evacuate anyone in need of help.

U.N.-appointed investigators estimate the death toll from the uprising at 6,400 civilians and 1,680 army defectors. Syrian government officials have said they only took military action when under armed attack from “terrorists.”