An initial blast was caused by a bomb planted under a car in the Al-Qosor district of the main city of Idlib province, the Syrian Observatory for Human Rights activist group reported.
After ambulances arrived at the site, a motorcycle bomb then detonated, the UK-based group said.
There was no immediate claim of responsibility for the attack, which the Observatory said also wounded at least 51, updating an earlier figure.
Among those injured were media activists Abou al-Baraa and Amer Zaidan, as well as a number of members of the Civil Defence, better known as the White Helmets.
“I was just passing that area when the attack happened. I drove near the area to help drive wounded to the hospitals but another attack happened right before I reach the street,” taxi driver Khaled Almayashe, told MEE from al-Qosor.
“I don’t know how to describe this tragedy and terror. Corpses, blood and body parts are all over the neighbourhood.”
“It’s heartbreaking,” he added. “I saw women sobbing for their sons and men, children next to their mothers laid on the ground.”
Ismael Abdullah, a member of the Civil Defence, said the attack took place near a school and relief organisation’s office, which were severely damaged. The whole area was in a “catastrophic situation”, Abdullah said.
“Many wounded have already been moved to Turkey to be treated. Most of the hospitals are filled with patients and can’t operate on badly wounded people,” he said.
Idlib, the last major part of Syria still outside the control of the government of President Bashar al-Assad, is held by an alliance led by Syria’s former al-Qaeda affiliate.
Hay’at Tahrir al-Sham (HTS) took control of the whole of the province last month, after defeating Turkish-backed competitors.
The Islamic State group also reportedly has sleeper cells in the area.
A local office of the self-styled “Salvation Government”, an administrative body created by HTS, is located on the street targeted by Monday’s attack.
Idlib has been protected from a massive government offensive since September by a buffer zone deal agreed by Damascus ally Russia and rebel-backer Turkey.
However, in recent days it has been hit by sporadic government shelling.
Government artillery and air strikes struck the towns of Khan Sheikhoun and Maarat al-Numan over the weekend, killing 10 on Saturday.
Further artillery and rocket barrages struck the towns on Sunday, killing one in Khan Sheikhoun.
Civil Defence and local activists said that the strikes had involved internationally prohibited cluster bomb munitions.
The two towns fall along a strategic highway through the northwest of Syria that connects provincial capitals Idlib, Hama and Aleppo.