* Withdrawal is part of complex deal with Assad's forces
* End of rebel presence in heart of "capital of revolution"
* Symbolic victory for Assad before his expected re-election (Adds quotes from opposition leader, paragraphs 4, 12-13)
By Dominic Evans
BEIRUT, May 7 (Reuters) - Hundreds of Syrian rebels on Wednesday abandoned their last stronghold in the heart of Homs city, an epicentre of the revolt against President Bashar al-Assad, handing him a symbolic victory weeks before his likely re-election.
Two convoys of buses snaked their way through the crumbling ruins of the besieged city, taking the fighters to safety in rebel-held areas outside the city under a deal agreed between the insurgents and forces loyal to Assad.
The Sunni Muslim fighters had held out in the Old City of Homs and neighbouring districts despite being undersupplied, outgunned and subjected to more than a year of siege and bombardment by Assad's forces.
"The people of Homs are real heroes. They have paid the bill for freedom," Syrian opposition leader Ahmad Jarba told PBS television in Washington. "The siege became lethal. We want the civilians and the fighters to get out of Homs safely so their lives can be saved."
At the same time the fighters were evacuated from Homs, dozens of captives held by rebels in the northern provinces of Aleppo and Latakia were also freed as part of the same deal.
But a planned relief convoy trying to reach two rebel-blockaded Shi'ite towns outside Aleppo was turned back by fighters from al Qaeda's Nusra Front, raising questions about the successful completion of the Homs operation.
Homs provincial Governor Talal Barazi denied reports during the day of any halt to the evacuation, which the Syrian Observatory for Human Rights monitoring group said had transported 900 fighters out of the old city.
Video footage of the first convoy showed a group of men climbing aboard a green bus, watched by around a dozen men in khaki uniform and black flak jackets marked "police". In front of the bus was a white car with the markings of the United Nations, which helped oversee the operation.
Activists said a total of 1,900 people, mainly rebel fighters, were being evacuated. The Syrian Arab Red Crescent said on its Twitter account it had sent ambulances to take wounded people out of the city centre.
Later video showed them arriving in a rebel-held area north of the city. Unlike what happened in an evacuation of civilians from Homs in February, activists said, they were not detained for checks by security forces and were allowed to keep their light weapons.
As the sunlight faded, it was not clear if the operation would be completed on Wednesday or continue into Thursday.
"This battle and this war has sometimes advances and sometimes retreats," Jarba, president of the Syrian National Coalition, told PBS through an intrepreter. "Sometimes we might lose ground, but our people, the Syrian people, are determined to win at the end and gain back the ground that we are losing."
During his Washington visit, Jarba will see U.S. Secretary of State John Kerry on Thursday and is expected to meet President Barack Obama.
The evacuation comes after months of gains by the army, backed by its Lebanese militant ally Hezbollah, along a strategic corridor of territory linking the capital, Damascus, with Homs and Assad's Alawite heartland on the Mediterranean.
The final rebel withdrawal from the centre of the city, known as the "capital of the revolution" when protests first erupted against Assad in 2011, would consolidate his military control ahead of a June 3 presidential election.
Assad is widely expected to be the runaway victor in the vote, which his opponents have dismissed as a charade.
They say no credible election can be held in a country fractured by civil war, with swathes of territory outside government control, 6 million people displaced and 2.5 million refugees abroad.
The fighters are expected to leave Homs in up to nine convoys, carefully synchronised with the aid delivery and the release of captives held by the rebels near Nubl and Zahraa, and the town of Kassab in Latakia province.
The anti-Assad Observatory said that captives included 29 fighters loyal to Assad but also 12 children and an Iranian woman. There was no independent confirmation. Iran has supported Assad in the three-year-old civil war.
More than 150,000 people have died in the conflict. Millions more have fled their homes and the government has lost control of stretches of territory across the north and east. Fighting regularly kills more than 200 people a day.
The provincial governor, Barazi, said Wednesday's operation would ultimately clear the whole of Homs city of gunmen and weapons, suggesting rebels would also be evacuated from the suburb of Al-Waer on the city's northwestern outskirts.
Rebels in Al-Waer and districts around the Old City have held out against Assad's forces since the army drove them out of the ruins of Baba Amr district, a cradle of the rebellion, in March 2012.
Since then, the army has gradually tightened its grip around the rebel areas, blocking weapons, medical supplies and food. It allowed hundreds of civilians to leave in February after lengthy U.N. mediation. (Additional reporting by Mariam Karouny and Oliver Holmes; Editing by Andrew Roche and Peter Cooney)