The Syrian army launched a massive assault on rebels in Aleppo on Saturday amid growing world concern about the risks of reprisals against the civilian population of the country's second city.
Troops backed by tanks and helicopter gunships, which had been massing for the past two days, moved on southwestern districts of the commercial hub, where rebel fighters concentrated their forces when they seized much of the northern city on July 20.
Artillery pounded Salaheddin and other rebel neighbourhoods from 8 am (0500 GMT) as ground troops advanced, an AFP correspondent reported.
Trapped civilians crowded into basements, seeking refuge from the intense bombardment.
"The fiercest clashes of the uprising are taking place in several neighbourhoods of the city," the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.
At least 10 soldiers and six rebels were killed in fierce fighting after the assault began, the Observatory said.
"The regime's forces tried to storm the headquarters of Salaheddin but thank God, the heroes of the (rebel) Free Syrian Army repulsed the attack," FSA Colonel Abdel Jabbar al-Oqaidi told AFP.
"We have now destroyed eight armoured vehicles," he said. "There are 100 tanks massed on the outskirts of the district.
"The battle will be hard because there is no balance of forces but we are determined and we have faith in God," he added.
The opposition fighters had been holding their fire in readiness for the threatened assault, the AFP correspondent said.
But their small arms and rocket-propelled grenades were little match for the heavy armour of President Bashar al-Assad's regime.
"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 calling himself Amer.
"There's a large number of civilians who have taken refuge in public parks." Pro-government media had warned that the "mother of all battles" loomed in Aleppo as the government moved to reassert its authority after recapturing rebel districts of the capital earlier in the week.
"Aleppo will be the last battle waged by the Syrian army to crush the terrorists and, after that, Syria will emerge from the crisis," the Al-Watan newspaper said.
Both sides acknowledged that casualties were likely to be high as the more than 16-month uprising comes to a head.
"Rebels are stationed in narrow streets, in which fighting will be difficult," a regime security official told AFP.
Nationwide, violence killed at least 52 people on Saturday -- 22 civilians, 16 rebels and 14 soldiers, the Observatory said.
On Friday, at least 148 people were killed -- 90 civilians, 24 rebels and 34 soldiers, according to the watchdog's figures.
Russia warned that a "tragedy" was looming in Aleppo but said it was unrealistic to expect the government would stand by when rebels were occupying major cities.
"We are persuading the government that they need to make some first gestures," said Foreign Minister Sergei Lavrov, whose government has long had close relations with Damascus.
"But when the armed opposition are occupying cities like Aleppo, where yet another tragedy is brewing as I understand... it is not realistic to expect that they will accept this," Lavrov told a news conference.
"Our Western partners... together with some of Syria's neighbours are essentially encouraging, supporting and directing an armed struggle against the regime," he added.
Turkey, which has given refuge to defecting army officers who have formed the kernel of the FSA, warned it could "not remain an observer" as the violence raged across its southern border.
"We must do what we can together in the United Nations Security Council, and also in the Organisation of Islamic Cooperation and the Arab League, to make sure that we can make some important progress in trying to avert this appalling situation," said Turkish Prime Minister Recep Tayyip Erdogan.
UN chief Ban Ki-moon called on Damascus not to press ahead with its attack.
"I'm seriously concerned by the escalating violence in Aleppo," Ban said. "I urge the Syrian government to halt the offensive." British Prime Minister David Cameron said there were "very real concerns that we have that the Syrian regime is about to carry out some truly appalling acts around and in the city of Aleppo." French foreign ministry spokesman Bernard Valero told AFP that "with the build-up of heavy weapons around Aleppo, Assad is preparing to carry out a fresh slaughter of his own people." In late May, at least 108 people were killed near the central town of Houla, the United Nations said. On July 12, regime forces killed more than 150 people in the central village of Treimsa, the Observatory said.