"We fear the government's retaliation -- may God help us," Ahmad, a resident of rebel-controlled Salahuddin neighborhood near the center of Aleppo, told The New York Times by Skype.
"People know there is going to be chaos, fighting, shelling, so people are frightened," an unidentified activist told the newspaper. "They have stocked up on canned goods and are not venturing out."
Parts of Aleppo are "a ghost city," another activist told the Los Angeles Times by Skype.
Hospitals anticipating bloodshed appealed for blood donations, the activist said.
While some residents of the nearly Chicago-size city of more than 2 million remained indoors, others fled, residents said.
Many of those fleeing were escaping for a second time, having run to Aleppo from the west-central opposition hubs of Homs and Hama, which the regime began attacking months ago, The New York Times reported.
Thousands of refugees have already sought shelter in Turkey, Lebanon, Jordan and Iraq.
The regime of President Bashar Assad dispatched nearly 80 tanks and hundreds of troops Wednesday from the neighboring Idlib province, opposition activists said.
Idlib rebel fighters 35 miles southwest of Aleppo told The Wall Street Journal they planned to exploit the sudden absence of regime forces to turn their area into a "safe zone" free of regime control.
Opposition fighters in Aleppo's key neighborhoods of Salahuddin, Sakhour and Shaar fortified their positions and ranks, preparing to face off with government troops residents said had encircled all three areas.
Rebels positioned heavy machine guns and anti-aircraft guns on rooftops in some areas, residents said.
Sporadic skirmishes were reported in restive neighborhoods before dawn Thursday in a sixth day of hostility in the city 30 miles east of Turkey.
Rebel strongholds were pounded Wednesday by Syrian army artillery, mortars and helicopter gunfire, the BBC reported. Ground troops periodically lobbed mortar shells.
Rebels claimed to have attacked and burned down several police stations Wednesday. A rebel video posted online showed rebels torching what appeared to be a police station. Thick black smoke appeared to billow from a building.
BBC video from Aleppo showed rebels setting up sniper positions in battered buildings, firing on helicopters with a machine gun mounted on a captured tank, and rounding up suspected regime collaborators.
Turkey, citing worsening security conditions, said Wednesday it would seal its border crossings with Syria, although the frontier would remain open to refugees fleeing violence.
In Damascus, regime officials acknowledged the Aleppo fighting 220 miles north.
But they said the relative calm that returned to the capital indicated the military knew how to control major cities.
"The army is doing its job there, like it did here," a ministry official told the Journal.
Violence, however, was reported in several Damascus neighborhoods, the Local Coordination Committees of Syria said.
The secretariat of the opposition Syrian National Council -- the biggest coalition of Syrian opposition groups, based in Istanbul, Turkey -- was to meet in Doha, Qatar, Thursday to discuss the formation of a "consensus-based" civilian administration.
Council sources told the British newspaper The Guardian Syrian political dissident and former Syrian Parliament member Riad Seif, a prominent businessman, was a leading candidate to head the administration.