Their palms facing the sky, around two million Muslims gathered Thursday on Saudi Arabia's Mount Arafat for the highlight of the hajj pilgrimage, one of the world's largest annual gatherings.

With temperatures pushing 40 degrees Celsius (104 Fahrenheit) under the desert sun, the faithful climbed the hill east of Mecca where Muslims believe the Prophet Mohammed gave his last sermon some 14 centuries ago.

They gathered in prayer on Jabal al-Rahma, or Mount of Mercy, for the second day of the hajj -- a five-day pilgrimage which all Muslims must perform at least once in their lifetime if physically and financially able.

Nada and Fida, two sisters from Syria, said they had dedicated their prayers to their home country, where more than 330,000 people have been killed and millions displaced since its civil war broke out in 2011. 

"I want Syria to go back to the way it was before," said Nada, 52.

"And for our youth to come back home," added her 47-year-old sister.

At sunset, a sea of pilgrims clad in white made their way down the hill and headed to Muzdalifa to spend the night.

On Friday, they will move on to Mina before sunrise for the hajj ritual of the symbolic "Stoning of the Devil".

This year's hajj has seen the return of pilgrims from Saudi Arabia's arch-rival Iran following a diplomatic row and a deadly stampede in 2015.

But thousands who would normally make the journey from neighbouring Qatar are absent apart from a few dozen because of a diplomatic crisis shaking the Gulf.

- All eyes on Mina -

"I came up here last night and prayed, took pictures and called my family and friends," said Maolana Yahia, 32, who made the trip from Indonesia.

Tunisian mother-of-three Fatima Arfawi said she was moved beyond words during the prayers on Mount Arafat.

"This is the first time I see anything like this, ever," she said. "This day is dedicated to prayer for my three children and my family."

In a hospital opposite the mountain, an area was set aside for people suffering dehydration or heat exhaustion.

Saudi Arabia's Red Crescent said it had deployed 326 ambulances along the pilgrimage route to handle health emergencies. 

"Some pilgrims, for example, forget to protect their heads with an umbrella when they pray," said Bandar al-Harthi, a nurse at a hospital facing Mount Arafat.

On Friday, the first day of the Eid al-Adha holiday, all eyes will be on the Jamarat Bridge in Mina, where the stoning ritual is held.

It was the scene of the 2015 stampede that claimed the lives of nearly 2,300 pilgrims, the worst disaster in the history of the hajj.

- Politics and pilgrimage - 

Tehran reported the largest number of victims in the 2015 stampede, with 464 Iranians among the dead.

The next year, amid heightened political tension between Saudi Arabia and Iran, Iranian pilgrims stayed away from the hajj after the two countries failed to agree on arrangements and logistics.

Iranian authorities say more than 86,000 Iranian pilgrims are taking part this year, each equipped with an identity bracelet in case of any accident.

Reza, a 63-year-old former oil company official from Iran, said he was torn between the joy of taking part and lingering grief over the stampede.

"They've taken more security measures otherwise we would not have come," he said.

Saudi Arabia says it has deployed more than 100,000 security personnel to keep pilgrims safe.

At the foot of Mount Arafat, mobile barriers had been installed to control the movement of the crowds.

Seated near the barriers, eight young women from Ghana who are all related took a brief respite after making their way from Mina.

Aged between 18 and 30, for some of them it was their first time away from home.

"This is the first time I leave Ghana," said 25-year-old Khadija. "My husband let me come alone because it is Mecca."