At least 107 people have been killed and 238 others injured after a crane collapsed in Mecca's Grand Mosque, Saudi Arabia's civil defence authority says.

Pictures posted online showed queues of Saudi men volunteering to donate blood for those injured in Friday's crash.

Al Jazeera's Hasan Patel, reporting from Mecca, said witnesses told him that a crane fell on the third floor of the Grand Mosque at around 5.30pm on Friday.

He said the mosque was packed even though the incident happened before the 6.30pm prayer.

"Dozens of ambulances headed to the site. The authorities closed off the area shortly afterwards," he said.

"This whole place is already a giant construction site. What made it worse is that around 5.30pm there was severe rain and it was just gushing down the road.

"I am surrounded by people who are grieving. The mood here is of sadness."

Major General Suleiman Al-Amro, director-general of Saudi's civil defence authority, told Saudi TV that a storm with severe rain and wind speeds as high as 83kmh caused the tower crane to collapse.

"The crane collapsed near Al-Salam gate on the upper side of Al-Masaa area and that caused the collapse of a small part of Al-Masaa and another section of Al-Mataf, the bridge area around the holy Kaaba,” Amro said.  

Pictures circulating on social media showed pilgrims in bloodied robes and masses of debris from a part of the crane that seemed to have crashed through a ceiling.

Richard Angwin, Al Jazeera's senior weather presenter, said the autumn period is when half of Mecca's thunderstorms occur so "it was no great surprise that there were thunderstorms in the area".

"The area of low pressure which brought sandstorms across the Levant region moved further south allowing hot, moist air to develop into thunderstorms along the mountains bordering the Red Sea, and some of those drifted northwards across Mecca," he said.

Mecca is Islam's holiest site. Pilgrims from around the world have been converging on the city and the mosque for the annual Hajj pilgrimage, which starts on September 21.

Muslim pilgrims move around the Kaaba, the cubic building at the Grand Mosque in the Saudi Arabian city of Mecca, on Wednesday [AP]

Source: Al Jazeera and agencies