Sheikh Ghassem said that there were no signs of religion, wisdom, or conscience in the terrorist attack.
He also urged certain Islamic countries, without naming them, to stop supporting terrorism.
On Monday night, masked gunmen stormed a mourning procession marking Ashura, the martyrdom anniversary of Imam Hussein (PBUH), the third Shiite imam, in the city of al-Dalwah in Eastern Province, and opened fire on mourners.
Seven Shiite mourners, including children, were killed in the terrorist attack.
The Saudi Arabian government claims al-Qaeda was behind the attack, Press TV reported.
Saudi Interior Ministry spokesman General Mansur al-Turki told the media on Wednesday that the attackers were “followers of the deviant ideology,” a term often used in Saudi Arabia to describe al-Qaeda.
The claim comes as there have been reports that Saudi Arabia has been supplying al-Qaeda-linked militants and other terrorist groups in Iraq and Syria with arms and money.
“Saudi Arabia has been financing al-Qaeda in Iraq, arming it through the back door,” Middle East expert Zayd al-Isa told Press TV in an interview in December 2013.