"It just started with a group of people pushing their way in and then finally they just got trapped and people started to step on bodies under their feet," one pilgrim told CBS Radio News.
The tragedy underlined the difficulty in managing one of the biggest religious events in the world, which this year drew more than 2.5 million pilgrims. It is the second deadly incident of this hajj pilgrimage, followingthat killed 76 people Jan. 5 in Mecca.
In the stoning ritual, all the pilgrims must pass by a series of three pillars called al-Jamarat, which represent the devil and which the faithful pelt with stones to purge themselves of sin.
The site in a desert plain of Mina outside Mecca is a notorious bottleneck in the week-long hajj pilgrimage and has seen deadly incidents in seven of the past 17 years, including a stampede in 1990 that killed 1,426 people and another in February 2004 that killed 244.
"I heard screaming and ... saw people jumping over each other," said Suad Abu Hamada, an Egyptian pilgrim, who was nearby when Thursday's stampede broke out.
"Police starting pulling out bodies. The bodies were piled up. I couldn't count them, they were too many," she said.
Afterward, bodies were lined up on the pavement nearby, covered with white sheets, and emergency workers rushed the injured away on stretchers. Police cleared part of the site, but thousands of pilgrims continued the stoning ritual nearby.
The Interior Minstry said 345 people were killed. State-run Saudi television Al-Ekhbariyah reported that most of the victims were from South Asia. The Health Ministry said 289 people were injured.
Since the 2004 stampede, Saudi authorities widened ramps leading to the platform where the three pillars are located and created more emergency exits to accommodate the crowds.
The small, round pillars were replaced with 26-meter-long walls to allow more people to stone them at once without jostling each other. The walls extend down through the bridge and protrude underneath, so pilgrims below can also carry out the stoning without going above.
Thursday's stampede occurred below the platform, near one of the entrance ramps.
"While the Saudi government tried very hard to provide the infrastructure, it's just simply an issue of overcrowding," Mehdi Hassan, a reporter with Sky News, told CBS News. "There are just too many people."
Thousands of pilgrims were rushing to complete the last of the three days of the stoning ritual before sunset, when some of them began to trip over dropped baggage, causing a large pileup, Interior Ministry spokesman Maj. Gen. Mansour al-Turki said.
Many pilgrims carry their personal belongings — such as tents, clothes or bags of food — with them as they move between the various stages of the hajj.