The rare infighting among the two groups added a new dimension of chaos to the civil war and heightened fears that if President Bashar Assad falls, the disparate factions battling the regime will turn against each other.
In the capital, a car bomb ripped through the main square of a residential neighborhood known as Mazzeh al-Jabal, killing 11 people and wounding dozens, the SANA state news agency said. The powerful blast caused widespread panic and massive destruction in residential buildings. Television footage showed bloodied people in the street as firefighters worked to put out the blaze.
"The explosion was very strong and broke all the windows in my apartment. I just ran with my son and daughter," said a woman in tears fleeing the blast site. "I don't know what happened to those who stayed behind, we couldn't see anything because of the smoke and flames."
The neighborhood is inhabited mostly by members of President Bashar Assad's Alawite minority, and residents said members of his security forces and military officers are known to live there, but it was not clear what the exact target was.
The car bomb, along with fierce fighting Monday in two other districts of the capital, was some of the worst violence in Damascus since July, when rebels took over several neighborhoods in the capital in a stunning attack. Within days, a regime counterattack pushed the rebels out of Damascus and recaptured the areas. Shortly after those battles, rebels moved on Syria's largest city, Aleppo, and it has become a major front in the civil war since then.
The attacks on Syria's two main cities since summer have demonstrated new organization and capabilities of rebel forces as well as a determination to press their uprising despite the deaths of more than 36,000 people in fighting over the last 19 months.
Rami Abdul-Rahman, who heads the Britain-based activist group Syrian Observatory for Human Rights, said the fighting in Damascus was concentrated in the southern neighborhood of Tadamon and the outskirts of the Palestinian refugee camp of Yarmouk.
Damascus-based activist Abu Qais al-Shami told The Associated Press via Skype that the fighting began Sunday night and went on continuously into Monday.
The Observatory and al-Shami said regime forces are backed by the Popular Front for the Liberation of Palestine-General Command (PFLP-GC) led by Ahmed Jibril, a radical Palestinian leader and staunch supporter of President Bashar Assad.
"Tadamon is being struck with shells, rockets and heavy machinegun fire," al-Shami said. "People are fleeing the area toward safer areas inside the Yarmouk camp."
A Syrian opposition figure, who asked not to be identified because of the sensitivity of the events, said Palestinian fighters who are opposed to Assad were fighting alongside the rebels in Damascus.
The Observatory had no immediate word on casualties from Monday's fighting but said eight people were killed in Yarmouk Sunday night when several mortar rounds landed in the camp.
When Syria's unrest erupted last year, the country's half-million Palestinians at first struggled to stay on the sidelines. But in recent months, many Palestinians began to the uprising.
The PFLP-GC, however, has remained loyal to Assad's regime.
The popular committees in Yarmouk Camp, which are led by the PFLP-GC and represent camp residents and Palestinian factions, said the camp was attacked by "terrorist gangs" who claim to include anti-government Palestinians.
"The mercenaries who claim to have Palestinians among them" tried to infiltrate the camp Sunday but were repulsed by the popular committees, the statement said. When the rebel attack failed, they fired mortars that killed and wounded several people, it added.
"Those who are shelling the camp are terrorists" who want to displace the Palestinians again, PFLP-GC spokesman Anwar Raja told The AP in Damascus.
Syrian authorities blame the anti-government uprising that began in March last year on a foreign conspiracy. They accuse Gulf countries Saudi Arabia and Qatar, along with the U.S, other Western countries and Turkey, of offering funding and training to the rebels, whom they describe as "terrorists."
In the central province of Hama, a suicide attacker detonated his car in the village of Ziyara causing some deaths, the state news agency said. It added that the blasts occurred outside a state-run development agency. The Britain-based activist group Syrian Observatory for Human Rights said the suicide attacker targeted an army checkpoint and killed at least 50 soldiers and pro-government gunmen.
The Observatory also reported that an air raid on the northern town of Harem killed at least 20 rebels. It said a rebel commander was seriously wounded in the raid.
Elsewhere in Syria, activists reported air raids, shelling and clashes in the northern provinces of Aleppo and Idlib near the Turkish border to Deir el-Zour region in the east and Homs in the center.
In the northern town of Kfar Nobol, the Observatory said an air raid killed and wounded a number of people. An amateur video posted online by activists showed cars and shops on fire and debris all over the street. A man was seen running carrying a dead body. The video appeared genuine and corresponded to other AP reporting on the events depicted.
In northern Syria, An opposition figure also said there were clashes between rival rebel groups for control of the Bab al-Salameh border crossing point with Turkey that has been in the hands of rebels since July. The opposition figure spoke on condition of anonymity because of the sensitivity of the matter.
The opposition figure said Sunday's fighting was between the Northern Storm Brigade and the Amr bin al-Aas brigade, which has a large number of Muslim hardliners.
There are dozens of opposition groups and rebel brigades fighting in Syria's civil war. Rivalries are common though violent clashes are unusual.
A Turkish government official based in the border town of Kilis confirmed two Syrian rebel groups were "engaged in a power struggle," fighting each other for the control of the Bab el-Salameh border crossing. The official, who spoke on condition of anonymity, in line with government rules, said however, that Turkish officials were still trying to determine who the two groups were.
Former Syrian Prime Minister Riad Hijab, who defected, met with Turkey's foreign minister behind closed doors in Ankara to discuss the Syrian opposition meeting in Qatar and efforts to restructure the opposition, a Turkish Foreign Ministry official said. The two also discussed the more than 110,000 Syrian refugees in Turkey.
It was Hijab's second visit to Turkey since he defected to Jordan in August.
Prominent Syrian-born Palestinian actor Mohammed Rafeh was buried in a Damascus cemetery a day after he was shot dead by anti-government gunmen for his apparent support of the regime.
Draped with the Syrian flag, Rafeh's coffin was carried into the cemetery on the shoulders of friends and relatives as men fired in the air in mourning. More than 500 men and women marched behind the coffin and later dispersed after the funeral procession ended.
Associated Press writer Suzan Fraser in Ankara, Turkey, contributed to this report.