0244 GMT July 23, 2019
The civil defence agency said on Twitter that emergency teams were sent to the scene after a "crane fell at the Grand Mosque," one of Islam's most revered sites, AFP reported.
That came about an hour after it tweeted that Mecca was "witnessing medium to heavy rains," and pictures on social media showed lightning.
Ahmed bin Mohammad al-Mansoori, spokesman for the two holy mosques, was quoted by the official Saudi Press Agency as saying part of a crane collapsed at 5:10 p.m. (1410 GMT) "as a result of strong winds and heavy rains."
Abdel Aziz Naqoor, who said he works at the mosque, told AFP he saw the crane fall after being hit by the storm.
"If it weren't for Al-Tawaf bridge the injuries and deaths would have been worse," he said, referring to a covered walkway that surrounds the holy Kaaba, which broke the crane's fall.
Pictures of the incident on Twitter showed bloodied bodies strewn across a courtyard where the top part of the crane, which appeared to have bent or snapped, had crashed into the building.
A video on YouTube showed people screaming and rushing around right after a massive crash was heard and as fog engulfed the city.
The incident occurred as hundreds of thousands of Muslims from all over the world gather for the annual Hajj pilgrimage expected to begin on September 21.
The Grand Mosque is usually at its most crowded on Fridays, the Muslim weekly day of prayer.
Many faithful would have been gathered there ahead of evening maghrib prayers, which occurred about an hour after the tragedy.
The governor of Mecca region, Prince Khaled al-Faisal, has ordered an investigation into the incident and was heading to the mosque, the official @makkahregion account on Twitter said.