President Bashar al-Assad dismissed criticism of Syria's presidential election as voters cast ballots Wednesday in government-held areas despite Western accusations it was neither "free nor fair".

"Your opinions have zero value," Assad said, after voting with his British-born wife Asma in the Damascus suburb of Douma, a key rebel stronghold until its recapture by government forces three years ago.

On the eve of the election, the United States, Britain, France, Germany and Italy said the poll was "neither free nor fair", and Syria's fragmented opposition has called it a "farce".

But they will watch powerless as few doubt Assad will trounce his two virtually unknown challengers to win a fourth term, in a war-battered country mired in economic crisis.

The controversial vote is the second election since the start of a decade-long civil war that has killed more than 388,000 people and displaced half the pre-war population.

Huge election posters glorifying Assad have mushroomed across the two-thirds of the country under his control.

State television showed long queues forming outside polling stations as voting got underway amid tight security.

Hundreds of students clamoured to vote outside Damascus University.

"I came to vote for Bashar al-Assad because he is the only man who withstood 10 years of war," said 26-year-old Kinan al-Khatib.

Around him, students chanted the slogan used to express support for strongmen across the Arab world: "With our blood and with our souls, we sacrifice our lives for you, Bashar."

Tolerated opposition 

The result is not expected before Friday evening, 48 hours after polls close.

The election takes place amid the lowest levels of violence since 2011 -- but with an economy in freefall.

More than 80 percent of the population live below the poverty line, and the Syrian pound has plunged in value against the dollar, causing skyrocketing inflation.

Assad's campaign slogan, "Hope through work", evokes the colossal reconstruction needed to rebuild the country, requiring billions of dollars in funding.

Assad, a 55-year-old ophthalmologist by training, was first elected by referendum in 2000 after the death of his father Hafez, who had ruled Syria for 30 years.

Standing against him are former state minister Abdallah Salloum Abdallah and Mahmud Merhi, a member of the so-called "tolerated opposition", long dismissed by exiled opposition leaders as an extension of the regime.

"I don't know the other candidates at all and I respect their nomination but my vote will definitely go to Assad," Khatib said.

Assad has refrained from holding campaign media events and interviews.

But he issued a general amnesty for thousands of prisoners this month, on top of a series of decrees aimed at improving economic conditions.

Last week, thousands of Syrians abroad with the right paperwork cast early ballots in their embassies.

But those who had fled the country illegally -- and so who could not show an exit stamp in their passport -- were barred from voting.

Several countries that oppose Assad blocked the vote altogether, including Turkey and Germany, which host large Syrian refugee populations.

No legitimacy

France's Foreign Minister Jean-Yves Le Drian said the vote was undermined by the fact that many Syrians at home and abroad were unable to take part.

"These elections are not real elections," he said.

The UN's Syria envoy, Geir Pedersen, said the polls were being held under the auspices of the current constitution and "not part of the political (transition) process called for in Security Council Resolution 2254" of December 2015.

"What is required is a Syrian-led and -owned political solution, facilitated by the United Nations and backed by constructive international diplomacy," he said.

In rebel-held northwestern Syria, home to three million people, hundreds of people took to the streets to protest the vote on Wednesday, an AFP correspondent said.

They carried posters saying: "No legitimacy for Assad and his elections."

Protestor Yaqin Beedo called the vote a "sham".

"It is a lie that regime loyalists actually believe," she told AFP.

Protests also broke out in Daraa, only parts of which are under the regime's military control and where some residents called for a boycott.

Kurdish authorities, who have carved out an autonomous zone in the northeast, said they were "not concerned". 

In the last multi-candidate poll in 2014, Assad took 88 percent of the vote.

It took place as the war raged, with the air force bombing rebel areas of second city Aleppo and fierce fighting in Hama, Idlib and Daraa, as well as near Damascus.