The march was organised by the Syria Solidarity Campaign.
The event’s Facebook page reads: “Besieged Eastern #Aleppo is on the verge of falling to the Assad regime.
“Reports estimate about 98% of Eastern Aleppo is now under the control of the Assad regime and its allies.”
The event’s Facebook page urged supporters: “Join us to protest against the large-scale bombardments and targeting of civilians in Aleppo.
“There is a Holocaust ongoing and we urgently need the international community to take action to save lives.”
As the march headed through Picadilly Circus, demonstrators could be heard chanting “drop aid not bombs”.
Other signs along the march read “protect civilians” and “Aleppo’s the new Holocaust”.
An agreement was reached on Saturday to allow “humanitarian cases” to leave two besieged government-held Shiite villages in northwestern Syria, a step that would allow the resumption of civilian and rebel evacuations from eastern Aleppo which were suspended a day earlier, the Associated Press reports.
The Britain-based Syrian Observatory for Human Rights said the evacuation of some 4,000 people, including wounded, from the villages of Foua and Kfarya was expected to start Saturday.
It later reported that 29 buses were heading toward the two villages to start the evacuation process, adding that insurgents in the area rejected allowing 4,000 people to leave and saying they will only allow 400 people to be evacuated.