Syrian regime forces have launched a ground assault in the city of Aleppo, and are bombarding opposition-held areas with heavy artillery, activists and witnesses have said.
Opposition forces say they have beaten off the first wave of attacks that began on Saturday morning, but that the battle continues.
At least 29 people were killed in Aleppo on Saturday, the Syrian Observatory for Human Rights (SOHR), a UK-based anti-government activist group, reported.
Civilians crowded into basements seeking refuge from the bombing, with the SOHR's Rami Abdel Rahman describing the clashes as the uprising's "fiercest".
"There are thousands of people in the streets fleeing the bombardment. They're being terrorised by helicopter gunships flying at low altitude," said an activist, adding many had taken refuge in public parks.
Battered and burnt-out tanks littered one of the main roads into the commercial capital, according to activists, who said the move marked the beginning of the expected regime assault on the city.
Syrian regime troops were focusing on the opposition-held neighborhoods of Saleheddine and Sakhour, the SOHR said.
'Armored vehicles destroyed'
Colonel Abdel Jabbar al-Oqaidi, a member of the Free Syrian Army, said that the revolution fighters had managed to repulse the regime's attacks on Saturday, redirecting them towards the neighborhood of Hamdaniyeh.
He said his forces had destroyed eight armored vehicles, and that 100 regime tanks were massed on the outskirts of Aleppo.
Abdel Rahman said the fact the soldiers had been stopped in Salaheddine "does not necessarily mean a withdrawal as their strategy is to bombard ... to cause an exodus then launch an assault even more fierce."
Saleheddine has been a stronghold of the opposition fighters since they seized large parts of Syria's second city on July 20.
Tanks, helicopter gunships and warplanes have poured into Aleppo over the last two days with troops firing on a string of neighborhoods in the battle to control the commercial hub of 2.5 million people.
In the surrounding countryside, however, the balance of power is changing fast. In an exclusive report from Al-Bab in Aleppo province, Al Jazeera's Anita McNaught said Free Syrian Army fighters have taken control while the Syrian army has largely disappeared.
"What we have been hearing from activists is that some neighborhoods in Aleppo have become battlefields, especially the neighborhood of Salaheddine in the southwest," Al Jazeera's Zeina Khodr reported from Beirut, in neighboring Lebanon, on Saturday.
"The battle for Aleppo really has begun. We have seen clashes over the past two days, and we have seen both sides send in reinforcements. It is really important for both sides to win, to take control of Aleppo, it's such a strategic territory."
Al Jazeera's Stephanie Dekker, reporting from Amman, in neighboring Jordan, said that activists in Aleppo were reporting that the opposition forces were holding strong in neighborhoods that they held, but that residents there were "terrified" of being killed if the army were to move in.
A medic in Aleppo, speaking on condition of anonymity, told Al Jazeera that parts of the city remained relatively calm, and that war was only focused on Saleheddine and other areas held by the armed opposition.
The heavy fighting around Aleppo and Damascus marked a new phase of the war that has gripped Syria since protests broke out about 17 months ago.
According to figures released by the SOHR on Saturday, more than 20,000 people have been killed since the uprising began in March 2011. That number includes 13,978 civilians and opposition fighters, 968 army defectors and 5,082 regime soldiers, SOHR Chief Abdel Rahman said.
PHOTO CAPTION
Syrians protest in the Shaar neighborhood of Aleppo during the funeral of a man killed in a bombardment by regime forces of Sukari, southwest of the restive city, on July 27.
Al-Jazeera