The United States has denounced Syrian leader Bashar Al-Assad's attempt to shore up his authority by staging presidential elections in the middle of a civil war.

"Today's presidential election in Syria is a disgrace," State Department spokeswoman Marie Harf said. "Assad has no more credibility today than he did yesterday."

Voting has now ended in the controversial election.

Polling stations opened at 7am in parts of Syria under Mr Assad's government control and remained open until after midnight local time following an extension.

"We hope for security and stability," said Hussam al-Din al Aws, an Arabic teacher who was the first person to vote at one polling station at a Damascus secondary school.

Asked who would win, he responded: "God willing, President Bashar al-Assad."

Mr Assad is running against two relatively unknown challengers who were approved by parliament to participate in the election, which his international opponents have dismissed as a sham.

The election is taking place more than three years after protests first broke out in Syria, calling for reform after four decades of Assad family rule. 

Authorities responded with force and the uprising descended into civil war.

Mr Assad's forces, backed by allies including Iran and Lebanon's militant group Hezbollah, have consolidated their control in central Syria, but the rebels and foreign jihadi fighters hold vast areas of northern and eastern Syria.

French Foreign Minister Laurent Fabius has described the election as "a tragic farce".

"The Syrians in a zone controlled by the Syrian government have a choice of Bashar or Bashar. This man has been described by the UN Secretary General as a criminal," he said.