The Makkah clock ordered by the Custodian of the Two Holy Mosques, King Abdullah ibn Abdul-Aziz, which is built at the fifth tower of the King Abdul-Aziz Endowment, began its experimental operation on the first minute after 12:00 noon the first day of the blessed month of Ramadan.
The clock project will be fully completed after three months from the start of the experimental operation.
The clock, which will loom over the Grand Mosque, is scheduled to enter service following a three-month trial period.
It boasts four glimmering 40 meter-across faces of high-tech composite tiles, some laced with gold, sitting more than 400 metres (1,320 feet) over the Holy Haram compound.
The tower's height will reach 601 meters.
Some 250 'highly qualified Muslim workers' were completing welding works on the clock's frame.
The clock's face is enshrined with the Arabic words 'In the Name of Allah' in huge letters underneath.
Some 21,000 white and green colored lights, fitted at the top of the clock, will flash to as far as 30 kilometers (18.7 miles) to signal Islam's mandatory five-times daily prayers.
The total weight of the clock amounts to 36,000 tons.