In a message on Thursday, Ayatollah Jannati offered his condolences to Leader of the Islamic Revolution Ayatollah Seyed Ali Khamenei as well as the Iranian nation, government and the families of the victims of the terror attack at the Shah Cheragh shrine in Shiraz.
He called on the entire nation to act in union in face of enemies' plots in order to be able to counter such "inhumane" acts.
The top cleric noted that those behind the terrible crime in Shiraz, for which the Takfiri Daesh terrorist group has claimed responsibility, should definitely face severe punishment.
The terror attack took place at around 5:45 p.m. local time (1415 GMT) on Wednesday just before the evening prayers.
The police commander of Fars province said the attacker opened indiscriminate fire at pilgrims inside the shrine and was himself wounded and arrested by security officials.
Reports say that the militant who carried out the attack was a foreign national.
CCTV footage released by local authorities showed the terrorist making his way inside the shrine and opening fire at people present there.