Horrifying video has shown corpses being loaded onto trucks and people passing out in the streets as more than 1,000 people have died amid severe heat at the Hajj pilgrimage in Saudi Arabia.

Worshippers have faced temperatures of more than 50C over recent days, with many succumbing to heat made worse by the huge crowds as some 1.8 million Muslims gathered for the annual event.

Pictures shared online appear to show bodies being covered and left in the street after they fell victim to the blistering heatwave, while many others suffered severe exhaustion and had to be urgently stretchered to safety.

Unregistered pilgrims make up more than half of those who perished, according to a count by AFP, with many unable to access air-conditioned facilities and help from authorities.

A British Imam described the scenes as 'harrowing' and said that it was impossible for pilgrims to walk through the streets and not to see people suffering from the scorching heat. 

Shocking video shared on social media shows trolleys carting scores of covered corpses to be buried, with what appear to be family members gathered around.

Officials in hi-vis jackets have also been pictured carrying pilgrims away on stretchers after they were unable to complete the lengthy route.

Elderly people were filmed struggling in the heat, with worried bystanders tending to those who were less able to deal with the extreme conditions.

In one video, a woman is seen surrounded by friends who are pouring water into her mouth in a bid to cool her down.

'There are so many stories from the pilgrims who are with me about what they saw in the streets, particularly last Sunday,' a British Muslim told the BBC.

'As you were walking, you could not walk 20 yards except you would see someone struggling in the heat. 

'People were doing whatever they could to shade themselves, they were taking water from kind individuals who were giving them towels, they were taking shade in hotel receptions.

'It was harrowing at times, I've never seen so many people struggle collectively.'

He went on to say that while authorities had been on hand to help, they need to learn lessons from the awful scenes caused by the extreme heat, made worse by overcrowding.

Around 1.8 million pilgrims took part in the pilgrimage to Mecca this year, 1.6 million of them from abroad, according to Saudi authorities. 

Each year tens of thousands of pilgrims attempt to perform the hajj through irregular channels as they cannot afford the often costly procedures for official hajj visas.

This places these off-the-books pilgrims at risk as they cannot access air-conditioned facilities provided by Saudi authorities along the hajj route.

'I think the Saudi authorities perhaps do need to start thinking about controlling the numbers of how many pilgrims come each and every year,' the British Imam said.

'With climate change, with the fact that the pilgrimage happens in the summer months, they really need to look into this in more detail.' 

The pilgrimage is increasingly affected by climate change, according to a Saudi study published last month that said temperatures in the area where rituals are performed were rising 0.4 degrees Celsius (0.72 degrees Fahrenheit) each decade.

Temperatures hit 51.8 degrees Celsius (125 Fahrenheit) at the Grand Mosque in Mecca on Monday, the Saudi national meteorology centre said.

Saudi officials had advised pilgrims to use umbrellas, drink plenty of water and avoid exposure to the sun, with curfews reportedly put in place at camps telling people not to leave their tents during the hottest hours of the day.

But many of the hajj rituals, including the prayers on Mount Arafat which took place on Saturday, involve being outdoors for hours in the daytime.

Saudi authorities reported clearing hundreds of thousands of unregistered pilgrims from Mecca this month, but it appears many still participated in the main rites which began last Friday. 

The new deaths reported include 58 from Egypt , according to an Arab diplomat who provided a breakdown showing that of 658 total dead from that country, 630 were unregistered.

One of the diplomats who spoke to AFP earlier in the week said that the Egyptian death toll was 'absolutely' boosted by a large number of unregistered Egyptian pilgrims. 

In the coming years the Hajj, the date of which comes 11 days earlier in the Islamic calendar every year, is set to fall in the winter, when temperatures are milder.

Pilgrim deaths are reported every year, not only due to heat but also the difficulty of the walk and the sheer amount of people who undertake it. 

The deadliest incident ever recorded at the pilgrimage was in 2015, when a stampede in the Mina camp killed over 2,400 pilgrims. Saudi Arabia has never acknowledged the full toll of the stampede. 

A separate crane collapse at Mecca's Grand Mosque, which preceded the Mina disaster, killed 111 people.

The second-deadliest incident at Hajj was a 1990 stampede that killed 1,426 people.