Clashes erupted in Syria's main cities of Damascus and Aleppo on Saturday, following a bloody day in which 132 people were killed nationwide, the Syrian Observatory for Human Rights said.

In Aleppo the army battled with rebels at the entrances to the rebel-held Bustan Al-Basha district and helicopter gunships attacked the opposition bastions of Hanano, Qadi Askar, Sheikh Khader, Bustan Al-Qasr and Sakhur, the monitoring group said.

After a week of fighting over the central district of Midan, the army had taken most of the area and set up checkpoints for the first time, an AFP correspondent said.

But the Observatory said that clashes between rebels and the army continued in the flashpoint district.

Residents of Midan had blocked off a contested street with tires and posted a small sign which read: "Beware of snipers," the correspondent said.

The Local Coordination Committees, a network of opposition activists on the ground, reported that an unknown number of people were killed or trapped under the rubble of a three-storey residential building that collapsed when the Sheikh Fares neighbourhood was shelled.

Regime forces also carried out air strikes against a rebel-held police station in Hanano for the second day in a row, after similar attacks on two police branches in Midan on Friday.

At least one rebel was killed in fighting in Aleppo, which has been the focal point of the Syrian conflict since mid-July, according to the Britain-based Observatory.

Outside Aleppo, air strikes during the night killed at least 12 civilians and wounded around 60 in Al-Bab, doctors in the rebel-held town told AFP on Saturday.

Two fighter jets carried out a series of raids on the town in northern Syria between 4 pm (1300 GMT) Friday and 4 am (0100 GMT) Saturday, hitting homes and empty school buildings, a doctor at the town's hospital said, asking not to be identified.

In one of the homes hit, four people lost their lives, three of them women, the father of one of those killed told AFP.

Near Damascus, a child and five rebels -- including a Palestinian defector from the Syrian army -- were killed by shelling and sniper fire in the southern suburb of Al-Hajar Al-Aswad where regime forces deployed in force, the Observatory said.

It said that a Palestinian protester was also killed during a demonstration in the Yarmuk camp, the largest Palestinian refugee camp in Syria.

Clashes broke out at the edges of the Damascus suburb, the watchdog said, while one civilian was killed in the northeast suburb of Douma and two civilians were killed elsewhere in the province.

In the southern province of Daraa, fierce clashes erupted between rebel fighters and government troops as the army tried to retake the Lajat area -- considered the most important rebel stronghold in the province.

In the central province of Homs, two civilians were killed in shelling on Qusayr, while five civilians were killed in shelling on the town of Qalaa al-Mazeeq in the central province of Hama, the Observatory said.

Three civilians, including two women, were killed in shelling in the northwest province of Idlib, while in the eastern city of Deir Ezzor, a 14-year-old was killed in shelling, while the town of Albu Kamal came under fresh bombardments, the Observatory said.

The Observatory gave an initial toll of at least 37 people -- 19 civilians, 9 rebels and 9 soldiers -- killed in Saturday's violence.

On Friday, 100 civilians, 13 rebels and 19 soldiers were killed nationwide in Syria, the Observatory said, noting that the largest single toll was in Daraa, where 12 civilians were killed in an air strike.