Yayladagi, Turkey (CNN) -- Syrian security forces and helicopters sprayed automatic weapons fire into a crowd of thousands of protesters demonstrating Friday after prayers in the northern town of Maaret al-Nouman, witnesses said.
At least four men were killed, activist Fadi Moustafa Sufi said.
An activist who has provided CNN with reliable information in the past said the crowd numbered in the tens of thousands when security forces on the streets and an attack helicopter aloft opened fire.
Some demonstrators used their personal weapons, including hunting rifles and AK-47s, to detain a number of members of the security forces when they ran out of ammunition, said the activist, who did not want to be named due to the sensitivity of the situation. He did not say that demonstrators exchanged fire with the security forces.
Citing the intensity of the gunfire from the security forces, the activist predicted the death toll would rise.
CNN cannot independently confirm the activist's report.
Another of the demonstrators, activist Yusuf Mohamed Ali Hassan, said he was shot in the right thigh by machine-gun fire from a helicopter.
"I'm at a friend's house getting treatment secretly from a doctor," Ali Hassan told CNN by telephone. "I didn't go to the hospital because I don't trust the hospitals."
The violence in Maaret al-Nouman erupted Friday after Syrian state television announced that the military had launched an operation to retake the nearby rebellious northern border town of Jisr Al-Shugur.
Meanwhile, Syrian refugees and opposition activists who had fled the town fearing a government attack said they heard tanks firing as they advanced through villages while approaching Jisr Al-Shugur.
"In response to the appeal from the families, units of the Syrian military began implementing its function to restore security in Jisr Al-Shugur and surrounding villages and arresting a number of militants," Syrian state TV announced Friday morning in a banner.
"They are attacking the village of Sirmaniya with tanks," opposition activist Ali Hassan said in a phone call to CNN earlier in the day, referring to a village less than 10 kilometers southwest of Jisr Al-Shugur.
This comes as anti-government demonstrators embarked on protests across the country, which has been engulfed in conflict between activists opposing the Bashar al-Assad regime and government forces working to clamp down on the outpourings.
Every Friday for weeks, anti-government marches have been organized after Muslim prayers. Friday's expression of discontent was dubbed "the Friday of kinship," denoting that all Syrians are part of one family.
The Syrian Observatory for Human Rights said at least 15 people have been killed in various locations, including Maaret al-Nouman.
From a hilltop near Sirmaniya, Ali Hassan said he saw black smoke rising from the village and heard tank cannon firing several times a minute.
The military advance spread panic throughout the civilian population. Residents say they had evacuated women and children from Jisr Al-Shugur in recent days. More than 3,800 Syrians have fled to Turkey, the semi-official Anatolia agency reported, and humanitarian workers feared many more were on the way.
"My family called (from Jisr Al-Shugur) and told me not to go back," said a 27-year-old man named Ali who had fled his hometown and was camped out along the border with Turkey.
The Syrian government announced it would punish Jisr Al-Shugur after it accused "armed groups" of killing at least 120 security forces there several days ago.
Refugees have disputed that claim. They say some soldiers rebelled after being ordered to open fire on unarmed protesters and started fighting among themselves instead.
Ali Hassan, the opposition activist, smuggled himself across the border to the Turkish village of Guvecci to speak Thursday night with journalists. CNN has been denied permission to report inside Syria.
Ali Hassan showed a video he said he had shot on his cell phone. It showed a column of thousands of unarmed male protesters walking near the town of Jabal al Zawiya, roughly 30 kilometers from Jisr Al-Shugur, on June 3.
Suddenly, long bursts of gunfire rang out, sending the demonstrators running and screaming for cover.
"This was a peaceful protest. The security forces started shooting at the people," Ali Hassan said. "They were 50 meters away from us. They were security officers wearing military uniforms. Some of them had beards and they didn't even speak Arabic."
Ali Hassan denied Syrian government accusations that the protesters were armed.
"I challenge anyone who says the protesters had guns or weapons," Ali Hassan said. "Nobody even carries a knife. I'm responsible for searching people who are going to demonstrate. Even if they have a wooden stick, we don't let them carry it."
Others making it over the border imparted similarly harrowing tales. One protester, shot last month in the leg and arm by Syrian security forces, was being treated at a hospital in Turkey. He said the security personnel opened fire at a peaceful protest one Friday and caused many casualties.
"We are oppressed and we want our freedom," said the 23-year-old protester, who didn't want to be named for security reasons.
A Syrian military officer said he and other troops defected because of the "massacre of the people of Syria."
"Our current mission is to protect unarmed protesters who are asking for freedom and democracy," he said.
The ongoing bloodshed in Syria is putting its neighbor and close trade partner Turkey in an increasingly uncomfortable position. On Thursday night, Turkey's prime minister, a political ally of the Syrian president, warned "we cannot keep supporting Syria in the face of all these developments."
Speaking on Turkey's ATV channel just three days before his party competes in parliamentary elections, Recep Tayyip Erdogan said he spoke four or five days ago with Assad.
"I explained this situation very clearly," he said. "Despite this, they take these matters very lightly.
"Syria is almost like an internal affair (for Turkey). We have an 800- to 900-kilometer border. We cannot close our doors to those who run for their lives and want to take shelter in Turkey."
He added, "How long can this continue?"
On Friday afternoon, at a refugee camp in the Turkish border town of Yayladagi, Syrian families chanted "down with the regime" and "thank you, Erdogan."
One man approached the fence of the abandoned tobacco factory where more than 1,500 refugees were being held and broke the rules imposed by the Turkish authorities to speak to CNN. He was crying and said all of his brothers were killed Friday at the protest in Maaret al-Nouman.
"We are protesting because of the massacre in Maaret al-Nouman," another man said.
A third man announced he would go on a hunger strike until the Assad regime collapses. Turkish police then shooed them away from the fence and the journalists.
Journalist Rasha Qass Yousef and CNN's Joe Duran and Arwa Damon contributed to this report