More than 2.5 million Muslims will on Friday begin the annual hajj pilgrimage in the Islamic holy city of Mecca, Saudi Arabia, 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 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 on 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 ultra-conservative kingdom.

The pilgrims will undertake religious rites that have remained unchanged since the founding of Islam 14 centuries ago.

"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. "I'm enjoying it here."

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, the Gulf's leading power, and its ally Washington accuse Iran -- Riyadh's regional rival -- 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, 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 degrees Celsius (104 Fahrenheit) 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. Traffic in the city is extremely heavy and the air quality is poor.

The odour of exhaust fumes mixes with the smell of bitumen emitted by the heated road surface and many pilgrims wear face masks.

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. But the vast gathering also presents major logistical and safety challenges. In 2015, some 2,300 worshippers were killed in the worst stampede in the history of the hajj.

Since then Saudi Arabia has sought to reassure pilgrims, deploying tens of thousands of security officers and installing cameras to oversee every corner of the holy sites.

Even though the hajj lasts only five days, many pilgrims arrive weeks before and linger in the city.

The total number of arrivals is limited however as each Muslim country receives a pilgrim quota.

Some worshippers have spent years on waiting lists -- sometimes as long as a decade -- just to perform their religious duty.

Outside of the hajj, Muslims can also undertake a lesser, year-round pilgrimage known as umrah.