* Wounded rebel leader was spearheading offensive
* Mortar hits stadium, killing soccer player (Adds 20 killed in Damascus suburb)
AMMAN, Feb 20 (Reuters) - A missile struck the command centre of the main Syrian rebel force near Damascus on Wednesday, wounding its leader, activists said, and a soccer player was killed when a mortar bomb hit a stadium in the centre of the capital.
The missile attack on Liwa al-Islam Brigade, which is spearheading a three-week-old offensive that has given the rebels a foothold inside Damascus, deals a blow to efforts to undermine President Bashar al-Assad in his seat of power.
A rebel spokesman said Sheikh Zahran Alloush, founder of the brigade, was wounded but declined to give further details.
"We cannot disclose Sheikh Zahran's condition," said the spokesman, Islam Alloush, who is also his cousin.
Activists said the early morning strike took place near the northern Damascus suburb of Douma. The rocket, possibly a Scud-type ballistic missile, devastated the area and killed or wounded other fighters, they said.
A rebel commander fighting with Liwa al-Islam in Damascus said: "It would be a great loss if Sheikh Alloush is killed. Liwa al-Islam is the most powerful on the ground and Sheikh Alloush is the brains behind its strength."
Activists and Syrian state media later reported a mortar bomb had struck the Tishreen stadium in central Damascus, killing a player from the Homs-based soccer team Al-Wathba.
Syrian television broadcast footage of what appeared to be the team's sleeping quarters, showing bloodstained walls and shattered glass on the beds.
The Syrian Observatory for Human Rights, which monitors violence in Syria, said at least 20 people were killed in an air strike on Damascus suburb of Hamouriya. (Reporting by Khaled Yacoub Oweis; Editing by Michael Roddy)