Makkah - More than two million Muslims from around the globe started the Haj pilgrimage on Sunday in Saudi Arabia, one of the world's largest annual gatherings in a country undergoing unprecedented change.

The ultra-conservative kingdom - where religion remains a guiding force amid dramatic social and economic reforms - has mobilised vast resources for the six-day journey, a pillar of Islam.

"It's the dream of every Muslim to come here to Makkah," said Frenchman Soliman Ben Mohri. "It's the ultimate journey. What worries me is the return to my normal life. For the moment, I am in a dream," the 53-year old told AFP.

Every Muslim is required to complete the Haj journey to Islam's holiest sites at least once in their lifetime if they are healthy enough and have the means to do so.

Tens of thousands of security personnel have been deployed for the pilgrimage, which was struck by its worst-ever disaster three years ago when around 2,300 worshippers were crushed to death in a stampede.

This year, the Saudis have launched a "smart Haj" initiative, with apps to help pilgrims with everything from travel plans to medical care.

The interior ministry said on Saturday that the number of pilgrims arriving in Makkah had already surpassed the two million mark, mostly from abroad including large contingents from Egypt, India, Pakistan and Bangladesh.

Wearing the simple white garb of the pilgrim, most of the faithful began moving on Sunday from Makkah to the nearby Mina Valley.

They will spend the night there in fire-resistant tents in the desert, where temperatures top 40 degrees Celsius (104 Fahrenheit).

Thousands of buses and vehicles carrying the pilgrims lined the eight kilometre road from Makkah to Mina.

Many pilgrims made the journey walking under the scorching sun, some carrying white or blue umbrellas.

For the Muslim faithful, Haj retraces the last steps of the Prophet Muhammad (PBUH) and also honours the prophets Abraham and Ismael (AS).

It ends with the Eidul Azha feast, which is marked by the slaughter of sheep, a tribute to Abraham's sacrifice of a lamb after God spared Ismael, his son.

Earlier in Makkah pilgrims performed Tawaf, a ritual walk seven times around the Holy Kaaba, a black masonry cube wrapped in a silk cloth embroidered in gold with Quranic verses at the centre of the Grand Mosque.

Holy Kaaba is the point towards which Muslims around the world pray.

"I feel so fortunate to be here," said Nazia Nour, 36, who came to Saudi Arabia from Auckland. Pushing her father on a wheelchair, she added: "I can't believe he (God) chose me."

On Monday, pilgrims will climb Mount Arafat for the climax of the Haj, praying and reading the Holy Quran.

Prophet Muhammad (PBUH) delivered his final sermon from the rocky hill to Muslims who had accompanied him on his final Haj, according to Islam.

After sunset, pilgrims head to Muzdalifah, halfway between Arafat and Mina, where they stay at least until midnight.

They gather pebbles to perform the symbolic stoning of the devil on the eve of the Eidul Azha feast.

Although the kingdom's young de facto leader, Crown Prince Mohammed bin Salman, has spearheaded change in the kingdom, religion remains a key force in Saudi Arabia.

The oil-rich kingdom ended its ban on women driving in June and has boosted female employment in a male-dominated society. But the reforms have been accompanied by a widening crackdown on dissent, with more than a dozen women's rights campaigners detained in recent weeks. Some have been released.

The Haj also comes more than a year into the worst political crisis to grip the Gulf, pitting Saudi Arabia against Qatar.

Saudi Arabia - the world's largest exporter of oil - and its allies accuse Qatar of cosying up to both extremists and Iran, Riyadh's main rival.

They have cut all ties with Qatar - which denies the charges - and banned all flights to and from Doha.

Qatar said Sunday that its citizens were unable to take part in the Haj because of the diplomatic dispute.

Saudi authorities have said Qatari pilgrims are still allowed into the kingdom for the Haj.