"We have come today... to reject normalisation... with this murderous, criminal, terrorist regime," Fahad Abdel Karim, 49, told AFP.
"We came to send a message to the whole world that with this normalisation, you will gain Bashar al-Assad the criminal, and you will lose the Syrian people," said Abdel Salam Mohammed Yussef, who heads a camp for displaced people.
Several hundred Syrians, some displaced from other parts of the country by the 12-year war, took part in the protest, according to an AFP reporter.
Assad has been politically isolated in the region since the war began in 2011, but a devastating February 6 earthquake that killed thousands in Turkey and Syria sparked Arab outreach.
A flurry of diplomatic activity has also been underway in past weeks as Middle East rivals Saudi Arabia and the Syrian government's ally Iran patched up ties, shifting regional relations.
On Tuesday, Saudi Foreign Minister Prince Faisal bin Farhan met with Assad in Damascus on the first trip by a Saudi official since the conflict began, less than a week after Syria's top diplomat Faisal Mekdad visited the kingdom.
Also this month, diplomats from nine Arab countries met in Saudi Arabia to discuss ending Syria's long spell in the diplomatic wilderness, while Mekdad visited Algeria, Tunisia and Egypt in a diplomatic push.
"We will never ever reconcile. What Saudi Arabia and the other countries are doing in terms of normalisation is nothing but an affront," said university student Hanifa al-Hammoud, 22.
"It's not their business, it's ours. This revolution is ours, it's not theirs."
Rebel-held Idlib is home to about three million people, around half of them displaced by the war.
The enclave is controlled by Hayat Tahrir al-Sham, the former Syrian branch of Al-Qaeda, and other rebel groups.
Some demonstrators held signs, including one that read: "Whoever forgives and reconciles (with Assad) is a criminal traitor... and is like him."
Syria's civil war broke out after Assad's repression of peaceful anti-government demonstrations escalated into a deadly conflict that pulled in foreign powers and global jihadists.
More than half a million people have been killed and around half of the country's pre-war population has been forced from their homes.
