Syrian President Bashar al-Assad oversaw a merciless crackdown on a pro-democracy revolt that morphed into one of the bloodiest wars of the century.

On Sunday, as rebels entered the capital, a Syrian war monitor said he had left the country, in what could spell the end not just of his 24-year rule but the downfall of his clan's five-decade reign.

After facing down nationwide protests demanding his ouster and an armed rebellion that he all but crushed, Assad had — until a lightning rebel offensive — taken back control of much of Syria in the civil war that began in 2011.

Quiet in demeanor, Assad had for years relied on his alliances with Russia, Iran and Lebanon's Hezbollah in order to maintain power.

While leading a merciless war of survival for his rule, he presented himself to his people and the outside world as Syria's only viable leader in the face of the Islamist threat.

But an Islamist-led rebel offensive that began on November 27 wrested city after city from Assad's control.

On Sunday the rebels announced they had entered Damascus.

Shortly afterwards, the Syrian Observatory for Human Rights said he had left the country, while the rebels themselves said the "tyrant" had "fled".

No real opposition

For years, Assad had cast himself as the protector of Syria's minorities, a bulwark against extremism and the sole possible purveyor of stability for a country ravaged by war.

In multiple votes held over the years, conducted solely on government-held territory, he took the vast majority of the ballots, amid accusations from Western countries and rights groups that the elections were neither free nor fair.

In official meetings, during interviews and even on the front lines, the 59-year-old ophthalmologist by training conducted himself calmly.

Behind the facade, however, was an astonishing ability to hold onto power amid multiple waves of violence and transformative change in Syria and the wider region.

'Shuffle the cards'

One journalist, who met with Assad on several occasions before and after war broke out in 2011, told AFP Assad is a "unique and complex figure".

Assad has "the same qualities" as his father, Hafez al-Assad, who ruled Syria for nearly three decades until his death in 2000, said the journalist, who declined to be named.

Hafez al-Assad, head of the Syrian Baath Party, imposed in the country a secretive, paranoid regime where even the slightest suspicion of dissent could land one in jail or worse.

Bashar al-Assad was never meant to become president, but his life changed radically when his older brother Bassel, who was being groomed to inherit power, died in a road accident in 1994.

Bashar quit his studies in ophthalmology and left London, where he had met his wife Asma, a British-Syrian and Sunni Muslim who worked for financial services firm JP Morgan.

Back home, he took a course in military studies and was tutored in politics by his father.

When the latter died, Bashar became president by referendum, running unopposed, then winning a second term in 2007.

Sworn in at the age of 34, Assad was initially seen by Syrians pining for freedoms as a reformer who could do away with years of repression and introduce economic liberalization.

In the early days, Assad would be seen driving his own car or having dinner at restaurants with his wife.

He relaxed some of the heavy restrictions that existed under his father.

Deadly crackdown

But his initial image as a reformer quickly evaporated as authorities arrested and jailed academics, intellectuals and other members of what was then known as the Damascus Spring movement.

When the Arab Spring reached Syria in March 2011, peaceful demonstrations broke out calling for change.

Assad, who was also commander-in-chief of the armed forces, responded by ordering a brutal crackdown on the protesters and civil war swiftly ensued.

Throughout the war, which killed more than 500,000 people and displaced half the population, Assad's position on the demonstrators and the opposition did not change.

To Syria and to the world, he justified the bombings and military campaigns as a war on "terrorists".

Meanwhile, his security apparatus enforced a brutal system of imprisoning dissidents in a network of detention centers and jails dotted around the country that have become notorious for abuses.

He was the subject of countless cartoons by dissident artists depicting him as a killer, not least in the aftermath of the 2013 chemical attacks on rebel bastions around Damascus.

Since the start of the Islamist-led rebel offensive, Assad has echoed his long-held stance that the conflict in Syria is machinated from abroad.

"The terrorist escalation reflects the far-reaching goals of dividing the region and fragmenting the countries in it and (to) redraw the map in line with the objectives of the United States and the West," Assad said on Monday.

He is the father of three children. His wife, Asma, was dubbed a "rose in the desert" by Vogue magazine before the revolt.