"Stop the massacre in Aleppo!", demonstrators chanted as they held placards saying "the bombing is not against Aleppo but our humanity" and "our brotherhood is besieged, not Aleppo".
The protest outside Istanbul University had been organised by religious student associations who called for "imperialist Russia to leave Syria".
In recent days, Assad's army has pressed a fierce assault aimed at retaking the whole of the city which is currently divided between the regime in the west and the rebels in the east.
The assault has left 42 children dead, from a total of more than 300 civilians killed since November 15.
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Moscow intervened militarily in support of Damascus last year but says it is not involved in the current assault on Aleppo. Turkey supports the Syrian opposition.
Protester Ramazan Kaya said Ankara should "raise its voice" against the regime offensive. "If we don't react, if we stay like observers of this massacre -- that is absolutely intolerable."
"We supported the Assad regime before he started killing his own people... But, at this stage, we have to be realistic that the person who kills almost 600,000 people should not rule any country," he said during a visit to Lebanon.
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