The 12th annual Pittsburgh Triathlon and Adventure Race, occurring this weekend on the North Shore, promises participants a visual experience along with an athletic one.

"It's unusual to hold a triathlon in the center of the city," said Thomas Baxter, executive director of Friends of the Riverfront, a nonprofit organization that benefits from the race. "When you're out on the water or biking down the HOV lanes, the city just opens up to you ... it blows competitors away."

Such a breathtaking view of Pittsburgh, he said, stuns hometown racers and attracts international participants from countries such as Germany, Australia and England.

Starting with the first competition at 6:45 a.m. Sunday, participants will attempt one of three different races either as individuals or as part of relay teams. According to Baxter, each race accommodates different skill levels and attitudes.

The most experienced and conditioned " tri-athletes" generally attempt the difficult International Distance Course, which requires a 1.5-kilometer swim on the Allegheny River, a 40K bike race on Interstate 279's high-occupancy vehicle lane and a 10K run along the Allegheny River Heritage Trail.

The Sprint Distance Course, a modified version of the advanced race, caters to lower endurance levels with a 60-meter swim, a 20K bike race and a 5K run in the same locations. It replaces last year's Heritage Trail Walk category and allows new racers to prepare for more difficult competition.

The third and final "Adventure Race" course, said Baxter, serves enjoyment seekers who may want to enter for fun with family and friends. Instead of swimming the first portion of the triathlon, Adventure Racers will paddle along a two-mile stretch of the Allegheny River in their choice of either a kayak or a canoe. Afterward, they will face a 20K bike race and a 5K run identical to those in the Sprint Distance course.

While all participants are awarded completion medals upon finishing their chosen courses, the top three winners in each category will receive a handmade trophy courtesy of Mendel Steel.

Baxter said this year's triathlon has drawn more than 700 registrations to date, which makes it the largest in event history despite the lagging economy. He attributes the unexpected increase in overall participation, including in the race's foreign representation, to marketing via social-networking technology.

He expects more than 1,000 spectators, who will gather atop Fort Duquesne and Clemente bridges for an ideal view.

Weekend festivities for racers, their families and the general public will begin with the second annual Triathlon Expo, held from 10 a.m. to 3 p.m. Saturday on the North Shore between PNC Park and Heinz Field. The health- and fitness-themed expo provides information about exercise and nutrition and offers attendees a chance to try out bikes used by triathlon pros.

All proceeds of the triathlon benefit the Friends of the Riverfront's mission to expand and maintain trails, said Baxter. In the past, funds have assisted in the construction of new paths and helped existing ones keep a safe and smooth surface.

Although online registrations closed yesterday, competitors may still print the application found at www.pittsburghtriathlon.org and return it until the day of the event. No registrations will be accepted the morning of the competition.