More than 100,000 security personnel are being deployed during the Hajj
The authorities in Saudi Arabia have warned people not to stage protests at the Hajj, as more than 2.5 million Muslims prepare for the pilgrimage.
The interior ministry official in charge of security, Gen Mansour al-Turki, said he did not expect trouble, but stressed that protests were banned.
"We will not allow any actions that might disturb any other pilgrims, or affect their safety," he told AFP.
In 1987, 402 people died when troops broke up a protest by Shia pilgrims.
Iran's President, Mahmoud Ahmadinejad, warned last month that it would take "appropriate measures" if its citizens faced restrictions.
The Iranian Supreme Leader, Ayatollah Khamenei, meanwhile said Shia should show they were addressing challenges to their unity.
Saudi officials and clerics responded by warning Tehran not to abuse the Hajj for political purposes.
This year is also the 30th anniversary of the seizure by Sunni extremists of the Great Mosque in Mecca, Islam's holiest shrine.
Sunni extremists seized the Grand Mosque of Mecca in 1979
Saudi soldiers, assisted by French paratroopers, took nearly two weeks to regain control of the complex. More than 250 people died in the siege, among them 127 Saudi troops. Sixty-seven militants were later executed for the uprising.
"We are taking all the countermeasures to make sure nothing like that could happen again," Gen Turki told AFP, adding that more than 100,000 security personnel were being deployed.
They have already thwarted several attempted attacks by al-Qaeda in the Arabian Peninsula, which is based in neighbouring Yemen.
The authorities are also hoping to avoid a repeat of the deadly stampedes which have in the past afflicted the Hajj. The last, in 2006, left 364 people dead.
Builders have just completed the rebuilding of the Jamarat Bridge at Mina, where pilgrims hurl stones at three pillars representing the spot where the devil is said to have appeared to Abraham.
Officials say the 950m (3,135ft) long, 80m (260ft) wide five-storey pedestrian walkway, which cost $1.2bn, will prevent overcrowding.