The protesters had set up roadblocks across the part of the M4 Highway that runs close to the government-held village of Nairab in the Idlib Province to oppose the patrols.
The casualties were caused after the Turkish military attacked them on Sunday, using live rounds and tear gas canisters, the Associated Press reported, citing activists.
The Turkish forces have been allowed to man a number of observation posts in Idlib as part of a de-escalation deal reached between Turkey and Russia in the Black Sea resort of Sochi in September 2018.
Earlier this year, however, Turkish troops were reported to have ventured far outside the areas that are assigned to the posts and engaged in heavy clashes with Syrian soldiers.
The Turkish presence in the province has also featured heavy support by Ankara for a number of anti-Damascus militant outfits, something that has been widely condemned as counterproductive to any de-escalation efforts.
Separately, the so-called Syrian Observatory for Human Rights, a UK-based monitor, said the Turkish military was implementing a policy of “demographic change” across several parts of Syria.
The policy has witnessed abductions and arbitrary arrests, it added, saying it was targeting Kurds and Arabs alike.
The Observatory named one of the targeted areas as the Kurdish-populated town of Afrin. It said civilians from Eastern Ghouta in southwestern Syria and other Syrian areas were also being housed in several areas in the northern cities of Raqqah and Hasakah.
The Turkish military launched its first incursion into Syria in 2016 to push back against Kurdish militants that Turkey associates with anti-Ankara separatists.
Ever since, the military has launched several other wholesale invasions into Syria. The people, whom the Observatory said were displaced by the Turkish forces, were targets of so-called Operation Peace Spring that Ankara waged against Syria late last year.