Heavy rainstorms have hampered the start of the annual Muslim pilgrimage, the Hajj, in the Saudi city of Mecca, BBC reported.
The authorities have warned pilgrims to take care in the rain at the event, which usually attracts about two million Muslims from around the world.
Fatal stampedes have marred the pilgrimage in the past, and many of the visitors stay in tents.
A senior Saudi official has also told the BBC that fears over swine flu may keep some 40% of local pilgrims away.
Four people attending the Hajj have so far died from the H1N1 virus, but officials have sought to play down the risk.
The BBC's Shahzeb Jillani at the Hajj says General Mansoor al-Turki's admission that numbers might be down this year is the first official recognition that fear of swine flu may affect the event.
The Kaaba inside the Grand mosque in Mecca
The six-day Hajj began with pilgrims circling the Kaaba
Health ministry spokesman Khaled Marghlani told AFP on Wednesday that only 67 pilgrims had been confirmed and suspected swine flu infections so far.
"Everything is going smoothly, thanks to God," he said.
Crowds of pilgrims have been converging on Mecca in recent days, and started the Hajj rites on Wednesday.
They began with Tawaf, walking seven times round the Kaaba - the cube-like building in the centre of the Great Mosque, Islam's holiest shrine - in an anti-clockwise direction.
They also ran seven times along a passageway in the Great Mosque, commemorating a search for water by Hajar, wife of the Prophet Abraham.
Pilgrims are now travelling to the valley of Arafat, where they will stand in the open praising Allah and meditating on Thursday.
'More cases detected'
The Saudi health ministry said the four people who had died from swine flu were a Sudanese man, a Moroccan woman and an Indian man - all aged over 75 - and a girl aged 17 from Nigeria.
Masked security official at Grand mosque
Security staff are taking swine flu precautions as pilgrims gather
A statement said the four had not followed "recommended procedures, especially vaccination against swine flu".
The hajj has been the scene of several tragic accidents caused by huge crowds.
In 2006, 364 people were killed in a stampede at the entrance to the Jamarat Bridge in Mina.
In a bid to avoid a repeat of the disasters, authorities have just completed the rebuilding of the bridge.
Officials say the 950m (3,135ft) long, 80m (260ft) wide five-storey pedestrian walkway, which cost $1.2bn (£723m), will prevent overcrowding.