* Officials predict large turnout as "political message"
* Opponents say no credible vote possible amid civil war
* Election follows military gains by Assad's forces
* France calls vote a tragic farce (Adds voting extended to midnight)
By Marwan Makdesi
DAMASCUS, June 3 (Reuters) - Syrians voted on Tuesday in an election expected to deliver an overwhelming victory for President Bashar al-Assad but which his opponents have dismissed as a charade in the midst of Syria's devastating civil war.
Rebel fighters, the political opposition in exile, Western powers and Gulf Arabs say no credible vote can be held in a country where swathes of territory are outside state control and millions have been displaced by conflict.
State television showed long queues of people waiting to vote at polling stations in areas under state control, as well as crowds waving flags and portraits of the president. Assad, looking relaxed and wearing a dark blue suit and light blue tie, voted at a central Damascus polling station with his wife Asma.
For many Syrians politics took second place to the overriding yearning for stability after three years of war which have killed more than 160,000 people.
"We hope for security and stability," said Hussam al-Din al Aws, an Arabic teacher who was the first person to vote at a polling station at a Damascus secondary school. Asked who would win, he responded: "God willing, President Bashar al-Assad."
Islamist insurgents battling to overthrow the 48-year-old president, who has ruled Syria since succeeding his father 14 years ago, dismissed the vote as "illegitimate".
But the Islamic Front and allied groups pledged not to target polling stations and urged other rebels to do the same.
Damascus residents said mortar shells struck residential areas in the capital on Tuesday, most likely fired from rebel suburbs. There were no immediate reports of casualties.
Assad is running against two relatively unknown challengers who were approved by a parliament packed with his supporters, the first time in half a century that Syrians have been offered a choice of candidates.
But neither of Assad's rivals, former minister Hassan al-Nouri or parliamentarian Maher Hajjar, enjoys much support.
"It's a tragic farce," French Foreign Minister Laurent Fabius said. "The Syrians in a zone controlled by the Syrian government have a choice of Bashar or Bashar. This man has been described by the U.N. Secretary General as a criminal," he told France 2 television.
But for many Syrians exhausted by war, particularly the minority Alawite, Christian and Druze communities, the Alawite president offers a bulwark against radical Sunni Muslim insurgents and the promise - however remote - of some form of stability.
At the Masnaa border crossing between Lebanon and Syria, thousands of people stood in the sun in a tightly packed queue to vote at a polling station set up for Syrians inside Lebanon - despite warnings from the government in Beirut that any refugees who crossed back into Syria would lose their refugee status.
All those who spoke to Reuters said they planned to vote for Assad, giving him a third seven-year term.
"I came and made the decision to do this for the sake of myself and my country," said Ghada Makki, 43. "It is a national duty to vote so that we overcome the crisis happening in Syria."
Some Damascus residents reported only a trickle of voters at polling stations in the centre of the city, but an activist who contacted people in Damascus and the Druze province of Suweida said the numbers of people voting was "scary".
"Lots of people have gone to vote and I'm not talking about the shabbiha," he said, referring to pro-Assad militia.
An hour before voting was due to end at 7 pm (1600 GMT), state media said polling stations would stay open until midnight because of what officials said was heavy voter turnout.
Foreign Minister Walid al-Moualem, draped in a Syrian flag as he voted, dismissed the foreign criticism. "No one in this world can impose their will on the Syrian people," he said. "Today the path to a political solution begins".
Information Minister Omran Zoabi, speaking on the eve of the vote, told Reuters the scale of the turnout would deliver a political message to Assad's foes.
"The armed terrorist groups have increased their threats because they fear (a high level of) participation," he said, referring to the rebels.
"If these terrorist groups had any popularity it would be enough to ensure the failure of the election," he said. "But they realise they have no popularity, so they want to affect the level of participation so they can say the turnout was low."
Tens of thousands of Syrian expatriates and refugees cast their ballots last week in an early round of voting, although the number was just a fraction of the nearly 3 million refugees and other Syrians living abroad.
The election took place three years after protests first broke out in Syria, calling for democratic reform in a country dominated since 1970 by the Assad family. Authorities responded with force and the uprising descended into civil war.
Assad's forces, backed by allies including Iran and Lebanon's militant group Hezbollah, have consolidated their control in central Syria but the insurgents and foreign jihadi fighters hold broad expanses of northern and eastern Syria.
Peace talks in Geneva between the government and the opposition National Coalition, which the opposition said must be based on the principle of Assad stepping aside in favour of a transitional government, collapsed in February.
Since then Assad's forces and Hezbollah fighters have seized back control of former rebel strongholds on the Lebanese border, cutting off supply lines for weapons and fighters, and the last rebels have retreated from the centre of the city of Homs.
The withdrawal from Homs has focused attention on the northern city of Aleppo, formerly Syria's commercial hub, where fighting has escalated in the last few weeks.
Rebel rocket fire on government-controlled areas of Aleppo killed 50 people over the weekend, while barrel bombs dropped by army helicopters on rebel-held areas of Aleppo have killed nearly 2,000 people this year, a monitoring group said.
State media said on Monday that a car bomb killed at least 10 people in Homs province. (Additional reporting by Alex Dziadosz, Laila Bassam and Mariam Karouny in Beirut and John Irish in Paris; Writing by Dominic Evans; Editing by Giles Elgood)