The death toll in Friday's crane collapse at Saudi Arabia's Grand Mosque in Mecca has risen to 87, with 184 more injured, Saudi officials said.

The exact cause it not yet known, but the incident — which happened after 5 p.m. local time — occurred amid a fast-moving storm featuring heavy rain.

The incident also comes as hundreds of thousands of Muslims began to gather from around the world for the annual hajj pilgrimage, which is set to begin later this month.

The Grand Mosque is usually at its most crowded on Fridays, the Muslim weekly day of prayer. The Saudi SPA news agency said in a statement that, as of Friday, almost 800,000 pilgrims had arrived into the kingdom for hajj.

Few other details were immediately available, but pictures circulating on social media showed bloodied bodies strewn across part of the mosque where the crane crashed.

The governor of Mecca region, Prince Khaled al-Faisal, has ordered an investigation into the incident.

Last year, the kingdom reduced the numbers permitted to perform the hajj for safety reasons because of construction work to enlarge the Grand Mosque. Much of the construction project, launched two years ago, had already been completed.

The massive project was undertaken to increase the area of the mosque by 4.3 million square feet, allowing it to accommodate up to 2.2 million people at once.

Al Jazeera and wire services