Al-Azhar's Grand Imam Ahmed El-Tayyeb warned members of different Islamist groups against accusing their opponents of blasphemy on Wednesday.
In a statement, El-Tayyeb said that Al-Azhar, Egypt's core Islamic institution, calls for agreement between different factions and warns against violence and attacking others' religious beliefs.
"This is against sharia [Islamic law]," the statement read.
Tensions have been on the rise in Egypt recently as the country anticipates nationwide demonstrations on 30 June aimed at pushing President Mohamed Morsi – who hails from the Muslim Brotherhood movement – to step down and hold early presidential elections.
In response to anticipated opposition protests, Islamist forces and government sympathisers are expected to hold "anti-violence" protests on 21 June.
El-Tayyeb criticised "false" religious fatwas by "incompetent" sheikhs who issue statements saying that those who oppose the ruler are "hypocrites and infidels" in the eyes of Islam.
The statement came following El-Tayyeb's meeting with President Morsi on Tuesday, in a joint visit with Egypt's Coptic Orthodox Patriarch Tawadros II to discuss the current political situation in Egypt.
The imam said that "peaceful opposition against [rulers] is accepted according to sharia… and has nothing to do with belief or lack thereof," adding that violence and militant actions are a "great sin" but not an act of "disbelief [kofr]."
Opponents of the groups calling for 30 June protests have criticised the protests, describing them as acts of hypocrisy that contradict Islamic rules and beliefs.
On Friday, Egypt's ultra-conservative group Al-Gamaa Al-Islamiya accused opposition groups calling for protests of being "entities being funded from abroad that aim to stir chaos and make Egypt like Syria or Libya," the group's spokesperson Mohamed Hassan said.