In just its third year, the Amgen Tour of California has already established itself as the strongest cycling event to be held on U.S. soil and one that figures to continue to grow in stature.

Last year, nearly 2 million spectators lined the streets and roads of cities throughout California to watch riders battle through a prologue and seven grueling stages, making it the most watched cycling event on U.S. soil. It's estimated that the race pumped more than $100 million into the state's economy.

This year's third annual event will feature 17 teams and 136 riders. It starts today in Palo Alto and concludes on Feb. 24 in Pasadena.

Over those eight days riders will compete across more than 650 miles of California roadway from Stanford University to Pasadena.

The sixth stage will take riders through through parts of Ojai and Santa Paula on its way to Santa Clarita.

Defending champion Levi Leipheimer, who has joined first-year team Astana, has a fierce group of competitors to fight off if he hopes to keep his title. The race has attracted an unprecedented field of riders from 27 countries, with cyclists hailing from countries as far away as Slovenia, Australia, New Zealand, Ukraine, Luxembourg, Kazakhstan and South Africa.

The field includes former world champions Paolo Bettini, Tom Boonen, Bradley Wiggins, Fabian Cancellara, Oscar Freire Gomez, Gerald Ciolek and Dmytro Grabovskyy. Former Olympic medalists Paolo Bettini, American Bobby Julich, Bradley Wiggins and Bradley McGee will also compete. Former national time-trial champions Vladimir Gusev, Edvald Boasson Hagen, Fabian Cancellara, Gustav Larsson, David Millar and David Zabriskie are also part of the field.

"We've never had a group this accomplished in our race and can't wait to see them out on the roads of California," said Andrew Messick, president of AEG Sports, the presenter of the race.

Leipheimer, who lives and trains in Santa Rosa, never trailed in last year's race. A three-time top-10 Tour de France finisher, he completed the 639.2-mile race with a 21-second advantage over Jens Voigt of Germany.

Leipheimer took the lead with a one-second win in the prologue in San Francisco. He maintained a lead of no more than three seconds until he dominated the individual time trial fifth stage in Solvang.

He said defending his title was on his mind all during the offseason.

"The Tour of California was a great motivator for me in the offseason," Leipheimer said. "It's one of the best races in the world."

Philippe Maertens, a spokesman for Leipheimer's new team Astana Cycling, said the team comes into this week's race with high expectations.

"Even though the new Astana Cycling Team is making its U.S. racing debut, we are bringing numerous riders who have found success on U.S. soil in the past," Maertens said. "The riders are excited to begin the season in California and are motivated to help Levi defend his Amgen Tour of California title."

Dirk Friel, a former professional cyclist who competed in Europe and Asia and co-founder of TrainingPeaks.com, said this year's Amgen Tour of California has bigger mountains to overcome, which will allow more of the climbers to take center stage as compared to last year's race.

Friel said there is only one stage that could be classified as less demanding than the 2007 edition. After cyclists faced a nearly all-uphill climb in last year's opening stage in San Francisco, this time around the 2.1-mile opener has been moved to a flat course in Palo Alto.

Stage 3 between Modesto and San Jose now includes an additional 40-mile stretch that rises from barely above sea level to an altitude of 4,360 feet atop Mount Hamilton.

In the final stage from Santa Clarita to Pasadena, riders will face the highest climb of the week up to the Millcreek Summit at 4,906 feet above sea level. Friel said the set-up could allow for some last-day heroics.

"The success of the Amgen Tour of California over the past two years has created a very positive reputation among the professional cycling community, which has helped to ensure the high level of competition that people have come to expect from our race," said Kristin Bachochin, senior director of AEG Sports.

"This year, we designed an even more challenging course and secured an incredible lineup of teams that promise to keep the level of excitement high for the fans and riders alike."