* Bombardment by rebels, troops shakes heart of Damascus
* Free Syrian Army founder loses leg in car bomb blast
* Moscow says Russia, China should join U.N. chemical inquiry (Adds footage of injured FSA founder, paragraph 13)
By Oliver Holmes
BEIRUT, March 25 (Reuters) - Syrian rebels lobbed mortar rounds into central Damascus on Monday, killing at least two people and drawing a fierce army response as bombardments shook the heart of the capital.
The state news agency said mortar bombs fired by "terrorists" had killed two people and wounded others near the Opera House on Ummayad Square, where Baath Party headquarters, Air Force Intelligence and state television are also located.
The military retaliated with artillery fire from Mount Qasioun, overlooking the city. "I've heard dozens of regime shells so far, pounding rebels," one resident said.
Photos posted by opposition activists showed black smoke rising from the square during what residents said was one of the heaviest bombardments in central Damascus since a revolt against President Bashar al-Assad erupted two years ago.
"The city is under attack," said one bewildered resident, adding that the explosions had begun at 6:30 a.m. (0430 GMT).
The conflict in Syria has killed 70,000 people and forced a million to flee the country, the United Nations says. Sustained fighting in Damascus could send thousands more into neighbouring states, especially Lebanon, which already hosts 370,000 of them.
There were no immediate reports that the rebels, who have pushed into the Kfar Souseh district, a few hundred metres (yards) from Ummayad Square, were trying to advance further.
Assad's forces have retained control of central Damascus and most other Syrian cities, while losing swathes of territory to insurgents elsewhere, especially in the north and east.
Colonel Riad al-Asaad, founder of the rebel Free Syrian Army (FSA), lost a leg in an overnight blast caused by a bomb placed beneath his car in the opposition-held eastern town of al-Mayadin, his deputy said. A Turkish official said Asaad was now being treated in Turkey and that his life was not in danger.
Asaad, who formed the FSA in 2011 as a popular uprising against Assad turned into an armed rebellion, was one of the first senior officers to defect from the Syrian military, but was sidelined last year from a new Western-backed FSA command.
Asaad's deputy, Malik al-Kurdi, told Al Jazeera television he believed the Syrian government had tried to assassinate the FSA founder with a bomb planted directly below his car seat. He said Asaad had also suffered face wounds.
No group has claimed responsibility for the attack.
Video footage posted online by activists showed Asaad lying on a bed with a bandage on his head and groaning. He was covered in a blue sheet and a man in the film said he would survive. Reuters could not independently verify the video.
REBEL FORCES DIVIDED
Various rebel units fight under the banner of the FSA, which has struggled to find weapons supplies and build a disciplined command and control structure. It does not include some Islamist militants such as the powerful al Qaeda-linked Nusra Front.
Moaz Alkhatib, who resigned on Sunday as head of the opposition Syrian National Coalition, said the attack on Asaad was part of a drive to "assassinate the free leaders of Syria".
Despite stepping down, Alkhatib said he would address an Arab League summit in Qatar this week. "I have decided to give a speech in the name of the Syrian people at the Doha conference," he wrote on his Twitter account on Monday.
Alkhatib, named leader of the coalition formed in November, is a Sunni Muslim cleric who had been seen as a moderate bulwark against the influence of al Qaeda-linked jihadist forces.
He resigned after the coalition berated him for offering Assad a negotiated deal and after the group went ahead, despite his objections, with steps to form a provisional government that would have diminished his authority.
The coalition is backed by Western powers and many Arab states, but Russia and China are critical of its insistence that Assad quit as a precondition for negotiations.
A senior Russian diplomat said on Monday his country wanted Russian and Chinese experts to take part in a U.N. investigation into charges that chemical arms were used in Syria on March 19.
U.N. Secretary-General Ban Ki-moon announced the inquiry on Thursday and made clear it would focus on a rocket attack that killed 26 people near Aleppo. Syria's government and opponents accused each other of firing a missile laden with chemicals.
Opposition activists said Syrian forces had used phosphorus on Monday when they fired multiple rocket launchers at fighters besieging an army base in Adra, a town near Damascus. Two rebels were killed and 23 wounded. The activists described it as another chemical attack. There was no independent confirmation.
U.S. and European officials say they have no evidence yet of any chemical attack. If one is confirmed, it would be the first use of such weapons by either side in the Syrian conflict.
Damascus has not confirmed it possesses chemical weapons, but says if it had them it would not use them on its own people. (Additional reporting by Jonathon Burch in Ankara, Khaled Yacoub Oweis in Amman, Steve Gutterman in Moscow and Ari Rabinovitch in Jerusalem; Writing by Alistair Lyon; Editing by Louise Ireland and Pravin Char)