Seven leading Islamist rebel groups in Syria say they are joining forces.
A statement posted online said Ahrar al-Sham, Jaysh al-Islam, Suqour al-Sham, Liwa al-Tawhid, Liwa al-Haqq, Ansar al-Sham and the Kurdish Islamic Front had agreed to a "gradual merger".
It said the new Islamic Front will be an "independent political, military and social formation" to topple the Assad regime and build an Islamic state.
The move comes as government forces make advances on key rebel-held areas.
In the past month, several towns around the capital Damascus and the second city of Aleppo have been recaptured by soldiers backed by pro-government militiamen, members of the Lebanese Shia Islamist movement Hezbollah and Iranian Revolutionary Guards, activists say.
An offensive has also been launched in the Qalamoun mountains, prompting thousands of refugees to flee into nearby Lebanon.
The formation of the Islamic Front also follows the death of Liwa al-Tawhid's military leader.
Abdul Qadir al-Saleh died on Monday of wounds he sustained in a government air strike on a rebel-held air base near Aleppo.
After Friday prayers, anti-government protesters are reported to have taken to the streets across Syria under the rallying cry "The blood of the martyr unites us".
At the same time, the seven groups announced that they were forming the largest rebel alliance yet in the two-and-a-half-year-old conflict.
"This independent political, military and social formation aims to topple the Assad regime completely and build an Islamic state where the sovereignty of God almighty alone will be our reference and ruler," a statement published on Facebook declared.
The rebels also outlined a new command structure, with key roles shared between leaders of the seven groups.
The Islamic Front's new leader will be Ahmed Issa al-Sheikh of Suqour al-Sham. He was previously head of the Syrian Islamic Liberation Front (SILF), a coalition of Islamist factions aligned to the Supreme Military Council of the Free Syrian Army.
The deputy leader will be Abu Omar Hureitan of Liwa al-Tawhid, while its head of military operations will be Zahran Alloush of Jaysh al-Islam.
The head of the political bureau will be Hassan Abboud. He is leader the Syrian Islamic Front (SIF), an independent coalition of hardline Islamist groups in which his own, Harakat Ahrar al-Sham al-Islamiyya, is the largest and dominant faction.
Liwa al-Tawhid spokesman Abu Firas told the AFP news agency: "The doors are open to all the military factions, and a committee is working to study the entrance of all groups that also want to join."
"It has been decided that all the factions' military, media, humanitarian and administrative offices will merge over a transitional period of three months," he added.
Charles Lister of IHS Jane's estimated that the Islamic Front's forces might command at least 45,000 fighters.
"This is an extremely significant development, both in terms of symbolism and the military effect it will likely have on the ground," he told the Reuters news agency.
Mr Lister said the Islamic Front would better represent forces on the ground and so undermine the exiled leadership of the National Coalition for Syrian Revolutionary and Opposition Forces.
The front may also challenge the growing influence of the two al-Qaeda-affiliated jihadist rebel groups, the al-Nusra Front and the Islamic State of Iraq and the Levant (ISIS), although they have co-operated with some of its component groups in the past.