The Syrian Civil Defence in Aleppo province, also known as the White Helmets, said its volunteers were able to remove at least 100 bodies from the scene of the blast.
Syrian state TV said the transfer resumed after the explosion, and dozens of wounded were taken to government areas.
Pro-Damascus media outlets said a suicide attacker had detonated a vehicle bomb and killed at least 22 people.
The convoy was carrying residents and pro-government fighters from the Shi'ite villages of al-Foua and Kefraya, which are besieged by rebels in nearby Idlib province, an insurgent stronghold.
The evacuation deal was brokered by Qatar, negotiating on behalf of the rebels, and Iran, on behalf of the government, in March.
Champions League: Milestone-man Ronaldo secures Real's comeback win over Bayern
German number one Manuel Neuer could count himself unfortunate to finish on the losing side following some superb saves. But Bayern were outplayed in the second-half and most of Madrid's 23 shots on goal came after the break.
The buses had been waiting since late on Friday outside the city while the evacuation deal halted. More than 20,000 people were bused out of Aleppo at the end of previous year, to rebel-held provinces in the northwest.
A large explosion targeted the buses, local sources told The New Arab, killing and injuring scores of fighters and residents from Foua and Kefraya, as well as rebels securing the convoy.
No one knows the exact number of the victims yet, the eyewitness said, claiming that "there are more dead people inside the buses".
They were allowed to leave the area in the "four towns" deal struck between rebels and Bashar al-Assad's government, which allowed thousands of people to leave the pro-rebel areas of Madaya and Zabadani, where residents had also been starving.
But a delay in the agreement had left all those evacuated stuck at transit points on Aleppo's outskirts since late on Friday.
Final day of campaigning before crucial Turkey referendum
Under the proposed changes, Erdogan could have his term limit effectively reset and stay in power through 2029. He also appealed to voters of other parties to approve the changes so "Turkey can leap into the future".
A resident of Zabadani, another rebel-held town to be evacuated, reportedly said that in last few days no evacuation has taken place from there.
"We consider it unacceptable to analyse events from a distance", he said after meeting his Syrian and Iranian counterparts.
The New York Times points out that "jihadist groups, like the Syrian affiliate of Al Qaeda, have a presence in the area and consider Shiites apostates who should be killed". They have been under siege by pro-government forces since summer 2015, leading residents to hunt rodents and boil grass to stave off hunger in the winter months.
Meanwhile, US-backed fighters reached the outskirts of a key ISIL-held town in northern Syria on Saturday as part of an offensive aimed at Raqqa - the de facto Syrian capital of ISIL's self-proclaimed "caliphate". They face daily bombardment at the hands of the government's air force in Idlib province.
More than 30,000 people are expected to be evacuated under the deal, which began on Wednesday with an exchange of prisoners.
White House decides against releasing visitor records
Under the new policy, records will be kept private for at least five years after Trump leaves the White House. By closing the site, the White House estimates it will save over $70,000 in taxpayer dollars by 2020.
He has been backed militarily by Russian Federation, and by Shiah fighters from Iran and the Lebanese Hezbollah group in Syria's six-year-old conflict.