The crane crashed into the Grand Mosque as it was full of worshippers, almost two weeks before the Hajj pilgrimage.
Officials say strong winds and heavy rains caused the crane to fall.
Correspondents say there had previously been concerns about safety on Saudi construction sites.
King Salman visited the site late on Saturday, and then met survivors of the accident in hospital.
"We will investigate all the reasons [of the collapse] and afterwards declare the results to the citizens," the official Saudi news agency quoted him as saying.
The victims were said to include worshippers from Iran, Turkey, Afghanistan, Egypt and Pakistan.
