By Erika Solomon

BEIRUT (Reuters) - Syria's Islamist rebels say the downfall of Egypt's popularly elected Muslim Brotherhood president has proven that Western nations pushing for democracy will never accept them, and reinforced the view of radicals that a violent power grab is their only resort.

Radical Islamist groups, some of them linked to al Qaeda, have lately been in the ascendancy in Syria's two-year conflict as the death toll rises above 100,000.

Assad has celebrated President Mohamed Mursi's fall as a symbolic blow to the Islamist-dominated opposition, though on the battlefield, where his troops are already making gains, it is likely to have little impact given the Brotherhood's limited role in the fighting.

Hardliners in the rebel ranks have long overshadowed the Muslim Brotherhood, a regional movement with a more moderate Islamist brand, and often criticized it for working within the framework of democracy instead of demanding an Islamic state.

"(We) always knew that our rights can only be regained by force and that is why we have chosen the ammunition box instead of the ballot box," said a statement by the Islamic State of Iraq and Syria, the local franchise of al Qaeda's international network, published on the day Mursi fell.

"If you want to shake off injustice and create change it can only be done by the sword. We choose to negotiate in the trenches, not in hotels. The conference lights should be turned off," it said, in an apparent reference to the Western and Gulf Arab-backed meetings for the Syrian opposition's National Coalition meetings in Istanbul this week.

There, Syria's own Muslim Brotherhood movement, which has dominated the political representation for the opposition abroad since its inception, also took a hit. For the first time, a non-Brotherhood president was elected, though that was expected even before the collapse of Mursi's government in Cairo.

PSYCHOLOGICAL BLOW

Pro-Assad groups were buoyed by the Egyptian army's removal of Mursi after millions protested against him. Assad, whose family has ruled Syria for more than four decades, called it "the fall of so-called political Islam".

Assad's father and predecessor put down an uprising by Syria's Muslim Brotherhood in the 1980s, killing thousands and leveling parts of the city of Hama in its suppression of the group's violent uprising there.

Since then, the Brotherhood has been nearly non-existent inside the country and became an expatriate movement.

But even pro-democracy Syrian activists say Mursi's fall has undermined their faith in Western and Gulf-Arab backed movements against autocratic leaders such as Assad and Mursi's predecessor Hosni Mubarak, who ruled Egypt for 30 years.

"Apparently armies in 'democracies' can topple presidents elected by the majority? This is a bad sign for revolutions," said Tareq, an Aleppo-based activist, speaking by Skype.

The Brotherhood's fall in Egypt will be felt by Syrian activists as more of a psychological blow than a strategic one. Activists say the Syrian branch is already learning from the regional movement's mistakes and had begun negotiating a new relationship with anti-Brotherhood regional powers such as Saudi Arabia.

But others say rebels may take more of a hit than they expect. One activist, who asked not to be named, said it could hurt the flow of weapons and cash coming from Libya as well as Egypt, where the erstwhile Brotherhood president had recently endorsed jihad, or "holy war" against Assad in Syria.

"Egypt was the transit point for some of those supplies and with the Egyptian army now in power, that might stop. And some Islamist groups that were providing us support will probably be more concerned with their own affairs," the activist said.

Joshua Landis, a U.S.-based Syria analyst, said if Mursi's downfall did weaken the Syrian Brotherhood, it would deprive the opposition of its only organized institutional force.

"Whatever our views of the Brotherhood, it was the only group that was organized and had structure. If it is weakened, all you will have is these small, extreme Islamist groups on the ground who can't gain support of Syria's middle class Sunnis," he said.

"NO MORE GAMES"

While Syria's Brotherhood had dominated the fractious National Coalition's negotiations with foreign powers for military and financial aid for the rebels, it never had a strong presence on the ground. A myriad array of mostly Islamist units along with army defectors run day-to-day affairs in rebel areas.

Many secular parties and groups that were loyal to autocratic leaders toppled by in the region's "Arab Spring" have celebrated what they see as a blow to Islamists who took power in post-uprising states such as Egypt and Tunisia. But radical groups fighting in unfinished revolts, as in Syria, say the ultimate outcome will be quite the opposite.

"Whether we supported Syria's Muslim Brotherhood or not, many Islamist rebels accepted that we might have to work within a civil state. Now, it is clear that world powers and their allies in the region are targeting Islam, first and foremost," said a rebel called Abu Nidal, from the Mustafa Brigades in Damascus.

"Now the Islamists reject any more of the international community's political games."

As Assad's forces gain momentum in the fight and continue to batter opposition areas with air strikes and artillery, many local opposition leaders have already turned their backs on the external opposition anyway.

They are frustrated at the inability of their political leadership abroad to create a unified front that could convince foreign powers to provide the opposition with more military and financial support.

"Muslim Brotherhood or otherwise, all these guys are becoming increasingly irrelevant," the analyst Landis said.

(Editing by Giles Elgood)