Hundreds of security forces with anti-riot equipment and backed by armoured personnel carriers and water cannons moved into the streets leading to the rally site, imposing a virtual curfew on the area, the Islamic Republic News Agency said.
The Saudis are crossing their fingers that no disaster will befall the annual haj pilgrimage to Mecca next week.
More than 1 million Muslims from around the world have gathered in Saudi Arabia for the haj. But the Saudis have also taken out insurance. It was at his request that the Iranian Foreign Minister, Ali Akbar Velayati, came to Saudi Arabia and the other member states of the Gulf Co-operation Council (GCC). Yet the Saudis were glad of an opportunity to express their hope of better relations, and no repetition of past troubles during the haj.
The Saudis, as guardians of the holy cities of Mecca and Medina, are acutely conscious of their responsiblilties to ensure the security of all pilgrims. The haj is one of the obligations imposed on all Muslims to perform once in their lifetimes, if they are able.
The concentration of so many people poses a serious challenge to the Saudi authorities. In the past they have accused the Iranians and the Libyans of using the haj to spread dissent and Islamic revolutionary fervour. In 1987, more than 400 pilgrims were killed, most of them Iranians, in clashes with Saudi security forces. For three years after that, Iran boycotted the haj. This year it is sending more than 100,000 pilgrims.
The haj was only one topic under discussion during Mr Velayati's week-long swing through the peninsula, which ended on Wednesday. In talks with leaders in Bahrain, Kuwait, Oman, Qatar, the United Arab Emirates (UAE), and Saudi Arabia, Mr Velayati stressed that Iran wanted detente.
The Gulf Arab states have historically been wary of their non- Arab neighbour, and what they see as its expansionist policy and interference in their internal affairs. There have been concerns about Iran's rearmament programme. And in the dispute between Iran and the UAE over the islands of Abu Musa and the Greater and Lesser Tunbs, the UAE obtained the backing of the GCC states.
But the Arab states believe in pragmatic accommodation with Iran, despite their differences. Hence the cordial welcome they gave their Iranian visitor.
This was all the more galling for the Egyptians. Egypt, like Syria, had hoped to capitalise on the support it gave the Arab states in the Gulf war against Iraq by providing some security for the GCC states against external threats. President Hosni Mubarak had, just before Mr Velayati's visit, been touring the same region warning the states of the threat posed to stability by what he called Iranian support for terrorism perpetrated by Islamic extremists.
The reception given to Mr Velayati suggested that the Arab states either felt Mr Mubarak was exaggerating the dangers, or that it was best to seek dialogue with the Iranians to avoid tensions. The leading foreign affairs commentator with the authoritative Egyptian daily al-Ahram, Salama Ahmed Salama, said the two visits, following so close, was a clear attempt by Iran to persuade the Arab Gulf states that there was a need to balance their relations with Egypt on the one hand, and Iran on the other.