More than 2.5 million Muslims will on Friday begin the annual hajj pilgrimage in Saudi Arabia's holy city of Mecca against a backdrop of tensions in the Gulf.

Crowds of worshippers have already begun to gather in Mecca in the days ahead of the hajj, the focal point of the Islamic ­lunar calendar. 

"It's the first time I've felt something so strongly - it's striking," said 40-year-old Indonesian pilgrim Sobar in rudimentary Arabic.

More than 1.8 million visitors had arrived by midday ­local time Tuesday, authorities said.

Crowds of faithful from across the world wore flowing white robes as they descended on the holy city located in the west of the kingdom. 

The pilgrims will undertake religious rites that have remained unchanged for centuries.

"Islam united us. We are all together... so that's why I'm very happy," said Leku Abibu, 46, a Ugandan mechanic who wore a beige salwar kameez. 

This year's hajj takes place amid tensions in the Gulf region exacerbated by a series of attacks on oil tankers, drone strikes and interceptions of maritime traffic on the high seas.

Saudi and its ally Washington accuse Iran of being behind the attacks and sabotage operations against commercial shipping. 

Tehran has denied responsibility.

Despite the absence of diplomatic ties between Saudi and Iran, some 88,550 Iranian pilgrims are due to take part in the hajj this year, according to the Tasnim News Agency.

Hajj is one of the five pillars of Islam and Muslims must perform it at least once in their lives if they can afford to.

"There are all nations of the world, all languages. There are no differences between us," said Nurul Jamal, a 61-year-old pilgrim from India.

Mecca, which is closed to non-Muslims, is home to the Kaaba - a cube structure draped in black cloth embroidered in gold located at the heart of the mosque.

The Great Mosque of Mecca with its Ottoman minarets sits amid skyscrapers that host ­upmarket malls and luxury hotels.

Muslims face toward the Kaaba when praying the prescribed five times daily.

Pilgrims circle the Kaaba seven times and perform a number of sacred rites.

While waiting for the beginning of the hajj, worshippers roam the esplanade or pray in the mosque in stifling heat. 

Temperatures can surpass 40 C and some pilgrims carry sun shades.

Large misting machines installed at the holy sites help to make the heat more bearable.

"The mosques are air-­conditioned, water is readily available, you just have to protect yourself from the sun," said Algerian Kamal Bouslimani, 57.

Men wear two pieces of unstitched cloth called "ihram" that leaves one shoulder exposed.

Pilgrims move together in large groups, led by guides holding their national flags.

Guards in khaki fastidiously lead worshippers to their five daily prayers.

Away from prayers, many of the faithful visit the innumerable shops that speckle the holy city. Religious tourism brings in revenues worth billions of dollars.