A Syrian activist group says troops opened fire on protesters after about 1,500 people took to the streets in the country's second city for an anti-government rally.

Wednesday's attacks on civilian dissidents led to clashes between the Free Syrian Army, the main armed opposition group, and government troops in the Aleppo's Fardous neighborhood, the activists said.

"In Fardous, demonstrations took place calling for the fall of the regime," the Syrian Observatory for Human Rights (SOHR) said in a statement sent to AFP news agency.

"The security forces opened fire on the demonstrators to disperse the protest."

The UK-based activist group said clashes broke out in the area immediately afterwards.

The SOHR did not have any details, however, on casualties resulting from the violence.

Also in the northern city, lawyers and other sympathizers of the opposition movement staged a sit-in on Wednesday at the city's judicial complex, according to activists.

"Aleppo's free lawyers are now staging a sit-in for the second consecutive day in the palace of justice, calling for the release of political prisoners, among them several lawyers," Mohammad al-Halabi, an activist, said.

More than 1,000 people were taking part in the demonstration, Halabi told AFP via Skype.

They were paying tribute to four Aleppo University students who were killed by government forces during a protest on May 3.

Syrian troops also tried to violently disperse the sit-in, and several of the protesters were detained, the SOHR said without specifying how many.

Syria's commercial center and traditionally a bastion for the government of President Bashar al-Assad, Aleppo has this month witnessed a major escalation in anti-government activity.

Meanwhile, the head of Syria's main opposition bloc, Burhan Ghalioun, formally resigned from his post, a statement issued by the Syrian National Council said Thursday after a two-day meeting in Istanbul.

The SNC "office decided to accept the resignation and to ask the council president to pursue his work until the election of a new president at a meeting on June 9-10", it said. Ghalioun announced his resignation on May 17 to avert divisions within the opposition bloc, after activists on the ground accused him of monopolizing power.

In southern Deraa province, more than 15,000 protesters rallied at a mass funeral for two people killed on Tuesday in the town of Dael, according to Rami Abdel Rahman, an SOHR activist.

More than 12,600 people have been killed in Syria since protests against Assad's rule broke out in March 2011, including nearly 1,500 since a UN-backed ceasefire deal took effect on April 12, he told AFP.

Al Jazeera could not independently verify the activists' accounts due to restrictions on reporting from inside Syria.

PHOTO CAPTION

United Nations observers on a monitoring mission in Syria are seen among members of the Free Syrian Army in Rasten, near Homs May 20, 2012.

Aljazeera