Syrian rebels in opposition-held areas engaged in fierce battles with Al-Qaeda-linked elements Friday in what activists say is growing resistance to the jihadists' brutal grip in many places.
Elsewhere in northern Syria, an unidentified group seized five people working for Doctors Without Borders (MSF) from a house, the Paris-based humanitarian organisation said.
Protesters turned out in rebel areas chanting the strongest slogans yet against the Islamic State of Iraq and the Levant, as tensions soar between the opposition and the Al-Qaeda affiliate.
Ammar, an activist on the ground, described it as "the start of the revolution against ISIL" which operates in Iraq and Syria.
Meanwhile a key group within Syria's mainstream opposition National Coalition stressed Friday that it will not attend peace talks scheduled for later this month in Switzerland.
"After meetings with many international delegations in recent weeks... the Syrian National Council confirms it sees no reason to attend the Geneva conference," SNC member Samir Nashar told AFP by telephone.
Nashar also forecast that the National Coalition, which has still not taken a definitive decision, would similarly not show up.
After months of delays, a January 22 date for the peace talks has been set, but doubts remain about whether the conference will go ahead.
The Coalition is set to hold its next general assembly meeting in Istanbul on Sunday and Monday.
The Syrian Observatory for Human Rights reported fierce fighting between rebels and ISIL in flashpoints of Aleppo city and province on Friday.
In Aleppo and nearby Idlib, 16 pro-Qaeda fighters were reported killed.
In Idlib alone, at least 42 ISIL fighters were wounded and 20 other civilians hurt in the crossfire, while in Aleppo, a media activist was killed while covering the fighting.
The Observatory and activists said a number of battalions united under the name "Army of Mujahedeen" and other rebel groups, including more moderate Islamists, were fighting ISIL.
The fighting comes two days after ISIL reportedly tortured and murdered Doctor Hussein al-Sleiman, known as Abu Rayyan.
His death was the latest in a string of beatings, kidnappings and killings that have enraged rebels and activists alike.
It prompted protesters to take to the streets under the slogan: "Friday of the martyr Abu Rayyan".
The Observatory and activists said ISIL fired on protesters in Aleppo city, who were chanting anti-regime slogans as they have every week since the outbreak of an uprising that has killed more than 130,000 people.
Both the Islamic Front and the Syrian Revolutionaries Front, two key alliances formed last year that group tens of thousands of fighters, condemned ISIL.
"We call on ISIL to withdraw immediately from Atareb... and remind them that those who freed Atareb (from Assad's regime) are those you are fighting today," said the Islamic Front.
Abu Leyla, an Idlib-based activist, told AFP via the Internet: "I'd say about 90 percent of people in the opposition areas are against ISIL.
"They use violence and abuses to crush dissent. They are only Islamic in name. All they want is power."
Syria's revolt began as a peaceful Arab Spring-inspired movement demanding the end of the Assad family's four-decade rule that was met with a brutal crackdown by the regime.
That sparked an armed uprising, and foreign jihadists soon flocked to Syria to join the rebels.
The jihadists were welcomed at first, but "their abuses have made it impossible for them to stay here. We want freedom, not ISIL," said Abu Leyla.
Meanwhile, five MSF staffers were taken from a house in northern Syria by an unknown group, "apparently for questioning," said Samantha Maurin, a spokeswoman for the international organisation.
It was unclear who had taken them, and MSF declined to release details about them or where they had been.
ISIL has been accused of targeting both foreign and Syria journalists as well as aid workers and activists for kidnapping.
In a separate development, Danish and Norwegian vessels left Cyprus and headed towards Syria to escort a delayed shipment of chemical weapons for destruction, said Norwegian army spokesman Lars Magne Hovtun.
The ships are to be joined by Chinese and Russian vessels inside Syrian waters.
The removal had been scheduled to take place before December 31, but the deadline passed and a new one has not yet been set.
The year-end deadline was the first major milestone under a UN Security Council-backed deal arranged by Russia and the United States that aims to eliminate all of Syria's chemical arms by mid-2014.