Over 1.6 million pilgrims arrived in Saudi Arabia for the largest religious gathering and a core requirement of the Islamic faith, which every able-bodied Muslim is expected to perform at least once in their lifetime.
The pilgrims are being housed in a tented encampment in the valley of Mina, five km (three miles) east of Mecca, the holiest city of Islam, and the Saudi government has deployed around 11,000 doctors, nurses and paramedics to provide medical care.
Iranian President Mahmoud Ahmadinejad is expected to perform the hajj this year at the personal invitation of Saudi Arabia's King Abdullah II and the first time the head of the Islamic Republic has ever taken part. The invitation is seen as a move to ease tensions between the predominantly-Shia Iran and the Sunni-dominated Saudi state.
This year the authorities have tightened security and over 50,000 soldiers have been deployed in the area to try and prevent casualties, which is a common occurrence. They have also added a third level to the Jamarat Bridge, where over 360 people died in 2006, to try and ease congestion. According to tradition, pilgrims throw stones at a pillar, representing Satan, from the bridge.
The five-day ritual begins each year on the eighth day of Dhul Hijjah, the 12th month of the Islamic calendar. Pilgrims have to reach the state of ihram - the ritual purification, and carry out all the sacred rituals correctly.
Male pilgrims have to wear special clothes - a garment consisting of two sheets of white cloth without any stitches, women are required to wear the traditional hijab.
After a day of prayers and meditation in Mina, pilgrims will head to the south, to Mount Arafat, where Muhammad is believed to have received the last passage of Islam's holy book, the Quran.
The hajj will reach its climax on Tuesday, when Muslims spend the day at Mount Arafat, praying and asking the God's forgiveness. On Wednesday they will descend from the mountain to celebrate Eid Al-Adha, a holiday to mark Prophet Ibrahim's readiness to sacrifice his son Ismail upon instructions from God.
Many Muslims believe that death during the hajj is a blessing. The highest death toll during the hajj was in 1990, when 1426 people were killed during a stampede in a Mecca tunnel.
Around 15-30% of Russia's 142-million population are Muslims according to different estimates. During the Soviet Union only 18 Russians traveled to Saudi Arabia for the hajj, which this year will cost Russian pilgrims between $2,000-3,000 and the only restrictions on numbers attending come from Saudi Arabia.
