The Muslim Brotherhood urged the Syrian people on Tuesday to rise up and back rebels locked in a "decisive battle" against the troops of President Bashar al-Assad in Damascus.
The influential Islamist group said the people must seize "this historic moment" by providing support for rebels who are fighting troops in several parts of Damascus for the third consecutive day.
"Prepare to become soldiers in the decisive battle. You will secure victory with your own two hands," a statement added.
The battle for Damascus must be "the gate to victory" in the 16-month uprising against Assad's autocratic regime.
"Our battle is now in Damascus ... and this requires that we mobilise all the forces and all our efforts to secure victory," the Brotherhood said.
The Brotherhood urged the people to hold peaceful demonstrations day and night against the regime and in support of the armed rebels.
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The statement follows the announcement late Monday by the rebel Free Syrian Army that it had launched a large-scale operation dubbed "the Damascus volcano and earthquakes of Syria."
Fighting has raged in Damascus since Sunday, with activists describing the escalation as a "turning point" in the uprising, with fierce clashes taking place in several neighbourhoods including Al-Midan close to the Old City.
The Brotherhood statement also comes as the organisation is due to wrap up a two-day congress -- the first in more than three decades -- on the outskirts of Istanbul, focused on ways of supporting the rebellion.
The Muslim Brotherhood was banned in Syria in 1963.
In the 1980s, it attempted to stir up a popular revolt against the regime of Assad's father and predecessor, Hafez, but the army crushed the uprising, killing between 10,000 and 40,000 people in the central city of Hama.
The Brotherhood is one of the key components of the Syrian National Council, the main opposition group in exile.