Alawite soldiers backed by tanks and helicopters recapture Midan district, but rebels attack other areas and border crossings.
Alawite troops and tanks on Friday drove rebels from a Damascus neighborhood where some of the heaviest of this week's fighting in the capital left cars gutted and fighters' bodies in the streets.
More than 200 people were killed in a single day, activists said on Friday, as the military struggled to regain momentum after a stunning bombing against the regime's leadership.
A fourth member of bloody dictator Bashar al-Assad's inner circle, national security chief General Hisham Ikhtiyar, died of wounds he suffered in Wednesday's bomb blast, which went off during a high-level security gathering in Damascus, according to regime's media.
The bombing has been described as a resounding blow to Assad, killing his defense minister and his influential brother-in-law along with another security official, all central to directing the crackdown on the uprising against his rule.
The blast, six days of sustained fighting in neighborhoods across central districts of the capital and the fall of several border posts into rebel hands appear to point to the unraveling of Assad's grip on power.
Regime troops regained control of the district of Midan in the southern part of Damascus on Friday. But rebels launched new fighting in several other districts of the capital, activists said.
Battles involving Alawites bringing in tanks, helicopters and mortars have turned parts of Damascus into combat zones, according to witnesses, and sent thousands of Syrian families packed in cars streaming across the border into neighboring Lebanon.
Damascus activist Khaled al-Shami said rebels carried out a "tactical" retreat early on Friday to spare civilians further shelling after five days of intense clashes between opposition fighters and regime forces.
Rebels continued to strike elsewhere in the capital on Friday. Rebels attacked a police station on Khaled bin Waleed Street, where heavy fighting was going on, the Britain-based Syrian Observatory for Human Rights said.
Clashes were also reported in the northern Barzeh and Rukneddine districts.
A Syria-based activist who goes by the name of Bashir al-Dimashqi said the rebels in Damascus were staging hit and run attacks and striking at security targets as opposed to controlling areas.
"Their strategy is to paralyze public institutions and chip away at the regime", he said.
Activists reported that 310 people were killed in violence nationwide on Thursday, making it the single deadliest day of fighting since the revolt began.
Besides the fighting in Damascus on Friday, about a half-dozen rebels took over a Syrian border crossing near the Iraqi town of Qaim on Thursday, said Iraqi puppet army Brigadier General Qassim al-Dulaimi. There are four major border posts with Iraq.
Rebels overtook a Alawite army outpost near the Syrian-Iraq border after clashes that killed 21 Alawite soldiers, he added.
In addition, amateur video posted online showed rebels taking over the Bab al-Hawa border crossing with Turkey, where they stomped on portraits of Assad.
A Turkish official based in Reyhanli, on the Turkish side of the border gate of Bab al-Hawa, confirmed that the rebels had taken control of the frontier crossing, but had no information on the latest situation over on the Syrian side.
Al Jazeera's Isil Sariyuce, reporting from Bab al-Hawa, said on Friday night that it was still "unclear" who was in control of the Syrian side of the border checkpoint.
"For the Free Syrian Army [the border crossing] is important strategically and symbolically. If they can control it, it will be easier to control the supply lines to their fighters, and it is the main transit point between Turkey and Syria".