A video shot by anti-government activists showed smoke hanging over Aleppo's center where the city's old central souk, or marketplace, along with medieval storehouses, shops, schools and ornate courtyards, are located, The New York Times reported.
"Our hearts and minds have been burned in this fire," said a physician in Aleppo, who gave her name only as Dima. "It's not just a souk and shops, but it's our soul too."
Rebel commanders had announced a major offensive Friday to secure control of the whole of Aleppo, the BBC reported.
The Syrian Network for Human Rights said it documented 78 deaths countrywide Saturday, with 52 of them in the Damascus area. Activist groups said 150 people were killed Friday, 40 of them in Aleppo. The casualty figures have not been independently verified.
"The sound from the fighting ... has been non-stop," said a resident of the central district of Sulamaniyeh, who identified himself as Ziad. "Everyone is terrified. I have never heard anything like this before."
The government, using artillery, tanks and aircraft, has a major firepower advantage in Aleppo and elsewhere in Syria, where rebels have waged an 18-month uprising against the government of President Bashar Assad, the Los Angeles Times reported.
"Today, God willing, is a decisive day in Aleppo. Just persevere, and follow the path of God," said a leader of the Al Tawheed Brigade, a major rebel unit.