Khaled Alameddin, a leading member of the Salafist Nour Party, has criticised former party leader Emad Abdel-Ghafour for his opposition to the arming of Syrian rebels.
The opinion of Abdel-Ghafour, who is President Mohamed Morsi's Social Communication Advisor, is opposed by most Sunni Islamists in Egypt.
"What happened to you Dr. Emad," said Alameddin via Facebook. "For God's sake, revolt for your beliefs."
Syrian President Bashar Al-Assad and many members of his regime are Alawites, an offshoot of Shia Islam. The majority of the opposition is Sunni.
The rift between Sunnis and Shias is the oldest in the history of Islam, sometimes sparking open sectarian conflict.
"Get angry over the blood of our [Sunni] brothers in Syria and the honour of our sisters that are being raped," Alameddin stated.
"Feel the pain of women, widows, orphan children and hundreds of thousands of injured."
During a meeting with Egyptian President Mohamed Morsi, leading Sunni Muslim clerics from various countries issued a call for jihad against Al-Assad and his allies, condemning the conflict as a "war on Islam," according to Reuter's news agency on Thursday.
Influential Muslim cleric Youssef El-Qaradawi, head of the Association of Muslim Scholars, along with the heads and representatives Islamic authorities in Arab and Islamic countries, attended the meeting.
The death toll in Syria since the beginning of the uprising in March 2011 has reached 90,000, according to a recent UN report. The opposition are believed to have lost more victims than their enemies.