Millions of Muslims from around the world have arrived in the Muslim holy city of Mecca to begin the annual Hajj pilgrimage. The four-day ritual is taking place under tight security, and officials in Saudi Arabia have made significant efforts to streamline the traffic flow to prevent a repeat of last year's deadly stampede. VOA Middle East Correspondent Challiss McDonough has more from Cairo.

An flood of pilgrims dressed in white robes have descended upon the Muslim holy city of Mecca and the nearby valley of Mina to begin the sacred rituals of the Hajj, which have been observed since the time of the Prophet Muhammad more than 1,400 years ago.

American pilgrim Hisham El-Khatib came from Los Angeles, California.

"I feel honored to be in this season, in this place and time, when so many people cannot afford to be here are not here for various reasons," he said. "It is a blessing to be able to attend the Hajj, and stand on Mt. Arafat like this."

The Hajj is one of the five pillars of Islam. Muslims are required to do it at least once in their lifetimes, if they can. The rituals include walking counter-clockwise seven times around the Kaaba, a large black stone cube that Muslims face when they pray.

The Hajj is the largest annual religious pilgrimage on Earth. Saudi officials are expecting some 2.5 million people from around the world this year, but the numbers can often be difficult to predict even though visas are tightly controlled.

Many first-time pilgrims are amazed to see the true diversity of Islam on vivid display.

"There are people from all over the world," Hisham El-Khatib said. "There are people who are from Africa, from Asia, far-away places in Africa, small villages. And you can tell that some people are overwhelmed by the crowd, they haven't seen such a huge crowd. Now, of course, I am from LA, and that's a huge gigopolis, and I'm used to crowds, but not like this. We are talking about [up to] three million Hajjis attending this pilgrimage, and you're going to see all of them at once. It is an amazing feeling."

Khatib is making his sixth pilgrimage, this time performing the Hajj rituals on behalf of his grandmother, who could not make it to Mecca. But for many Muslims, this is a once-in-a-lifetime experience - something they spend years saving and preparing for.

Khatib is traveling with a group of pilgrims from Southern California who have been studying for weeks, taking "Hajj classes" taught by an Orange County imam. Saudi authorities encourage these educational sessions so first-time pilgrims know the proper procedures - both for meeting the religious requirements of the Hajj, and for making the pilgrimage safer for everyone.

With the sheer volume of pilgrims, crowd management and logistics are enormous challenges for the Saudi authorities. There have been Hajj tragedies in the past.

Just last year, a stampede killed 364 people at the entrance of the Jamarat bridge in Mina, where pilgrims must throw stones at three pillars that symbolize the devil. This year, Saudi authorities have tried to ease the traffic flow in that area by adding a third level to the bridge.

Khatib was in Mecca during another deadly stampede in 2004. He says the improvements made since then are clear.

"Definitely, definitely," he said. "I see improvements every year."

Among the pilgrims this year is Iranian President Mahmoud Ahmadinejad. He was invited by Saudi King Abdullah and arrived in Medina on Monday. It is the first time, at least in the modern era, that a sitting Iranian president has performed the pilgrimage while in office. Relations between Shiite Iran and Sunni Saudi Arabia have been tense in the past, and analysts view the king's invitation as a sign that the two countries are trying to improve their ties.