Hundreds demonstrated throughout the territory, the Syrian Observatory for Human Rights said, following a rampage against Syrian businesses and properties in central Turkey where a Syrian man had been accused of harassing a child.
"Armed demonstrators and Turkish forces engaged in armed clashes... in the city of Afrin," in Syria's Turkish-controlled north, said the Britain-based monitor, which has a network of sources inside Syria.
"Four people were killed in exchanges of fire between protesters and guards stationed at Turkish positions," the Observatory said later, having first reported one death.
Three of the dead were killed in the city of Afrin and another in Jarablos, with 20 more injured, the Observatory added.
An AFP correspondent in the northern city of Azaz said dozens protested there, with some raising the flag of the Syrian opposition.
Armed gunmen shot at Turkish trucks in the nearby city of Al-Bab, the correspondent said.
Demonstrations "accompanied by acts of violence" spread in "large areas" of the Turkey-controlled border strip, Observatory chief Rami Abel Rahman told AFP, with protests also extending to parts of the nearby rebel-held Idlib region.
Since 2016, Turkey has carried out successive ground operations to expel Kurdish forces from border areas of northern Syria.
Pro-Turkish forces in Syria now control two vast strips of territory along the border.
Some protesters took down Turkish flags, with border guards firing at demonstrators who attempted to storm the Jarablus border crossing, the Observatory said.
Protester Adel al-Faraj told AFP he took to the streets in solidarity with "our Syrian brothers in Turkey".
"Our people fled from (Syrian President) Bashar al-Assad only to be oppressed in Turkey," he said, urging Ankara to do more to stop violence against Syrians.
On Friday, Erdogan had pointed to a possible meeting with Assad, saying it was "not impossible".
-'Unacceptable'-
Earlier on Monday, Turkish police held 67 people after the rampage in Kayseri, with Erdogan condemning the latest bout of violence against Turkey's large community of Syrian refugees.
Videos on social media showed a grocery store being set on fire.
"No matter who they are, setting streets and people's houses on fire is unacceptable," Erdogan said, warning that hate speech should not be used for political gains.
Interior Minister Ali Yerlikaya said the Syrian national accused of harassing a child, identified only by his initials as IA, was caught by Turkish citizens and delivered to the police.
A court in Kayseri ordered the Syrian man's arrest late on Monday, Justice Minister Yilmaz Tunc said, adding: "The child and her family are under our state's protection".
Yerlikaya said on X that the child was a Syrian girl, who was a related to her harasser.
He said Turks who gathered in the area acted "illegally" and in a manner "that does not suit our human values", damaging houses, shops and cars belonging to Syrians.
Sixty-seven people were detained after the attacks, he said.
In one of the videos, a Turkish man was heard shouting: "We don't want any more Syrians! We don't want any more foreigners."
Turkey, which hosts some 3.2 million Syrian refugees, has been shaken several times by bouts of xenophobic violence in recent years, often triggered by rumours spreading on social media and instant messaging applications.
