LEWISTON — As TV celebrity Patrick Dempsey counted down to start time, hundreds of runners and walkers wearing a kaleidoscope of colorful T-shirts surged down Oxford Street early Saturday morning to launch the Dempsey Challenge of 2011.

A noncompetitive run/walk of 10 kilometers opened a full weekend of events based at Simard-Payne Memorial Park. By mid-morning, the third annual Dempsey Challenge had registered about 3,800 people of all ages in the fundraising effort to benefit the Patrick Dempsey Center for Cancer Hope & Healing at Central Maine Medical Center. Pledge totals were approaching $900,000. The warm and sunny weather was expected to bring out many more participants before the end of the two-day event, event manager Aimee Arsenault said.

Dempsey, who portrays Dr. Derek Shepherd on TV’s “Grey’s Anatomy,” drew cheers at his appearances and participation throughout the opening day. He founded the Dempsey Center in 2008 in response to a need for a cancer patient support facility following his mother’s successful fight against a diagnosis of ovarian cancer.

“I think the success of the center is based on all the volunteers,” Dempsey said at a 9 a.m. news conference at the Hilton Garden Inn in Auburn.

It’s a team effort, he said.

“My mom is now volunteering there, which is really great to see,” Dempsey said. “It’s great to see so much life going on,” he said, noting that families of the L-A area deserve much credit for devoting their time and money to the Dempsey Challenge, despite difficult economic circumstances.

“It was wonderful to come back to my hometown,” he said, “and it was nice to bring my daughter and be able to point things out and tell her that’s where I was when I was a kid.”

Dempsey and a group of nine nationally known professional cyclists rode privately through the local countryside Friday, and Dempsey said that he was pleased to see so much downtown redevelopment.

He and the pro cyclists will take part in Sunday’s rides, which will cover routes of 10, 25, 50, 70 and 100 miles.

“Cycling is a great metaphor for teamwork,” he said. He thanked each of the pro cyclists for coming to Lewiston, and he voiced special appreciation to Levi Leipheimer, Olympic bronze medalist and three-time Tour of California Champion, for the opportunity to ride with him last week for a community fundraising event that Leipheimer supports. Leipheimer also was in Lewiston for last year’s Dempsey Challenge.

As the crowd grew in front of the announcers’ stage at the Oxford Street starting line, Dempsey moved freely to the railings of the course. He signed autographs and posed for dozens of pictures, chatting easily with participants.

Organizers said 361 teams would take part in the running, walking and cycling throughout the week. Many teams wore identifying T-shirts for the 10K and 5K run/walk sponsored by L.L. Bean. Among the names were “Team Semper Fi,” “I Run For Mimi,” and “Dempsey Dudes and Dames.”

Laura Davis, who was named the 2011 Amgen Breakaway from Cancer Survivor Award-winner, delivered deeply moving remarks from several stages Saturday. The 22-year cancer survivor from Buckfield talked about how she has faced the dreaded disease seven times. Her first diagnosis came just four weeks after the birth of her first daughter.

Davis called for moments of silence to remember the stories of those who could not be present, and said, “Sometimes, those stories are best shared in silence, rather than in speech.”

Dempsey, his mother, Amanda Dempsey, his sister, Mary Dempsey, who is the assistant director of the center, and Davis led Saturday’s Cancer Survivor Walk at Simard-Payne Memorial Park when dozens of survivors marched along a portion of the park’s perimeter.

Another first-day event was a kids’ run. Youngsters beginning at elementary school age dressed in all kinds of superhero costumes to represent the fight against cancer.

This year’s Dempsey challenge introduced a program called Positive Tracks aimed at youths, in which money raised by people 23 and younger will be matched dollar for dollar, up to $100,000, by a Challenge Grant for Youth program funded by organizations throughout New England. The money will go to the center’s Healing Tree Children’s Program. More than 600 young people had enrolled in the program by noon Saturday.

The Dempsey Challenge site between Lincoln Street and the Androscoggin River was packed with visitors through the day. About 40 vendors were set up, as well as a large food tent run by the Green Ladle culinary arts program at Lewiston Regional Technical Center where participants were served omelets, pizza, vegetarian chili and other dishes. Director and Chef Dan Caron said about 2,000 omelets were prepared on the spot Saturday morning. Lobster is on the menu for Sunday.

Peter Chalke, CEO of CMMC, told attendees that every cent raised by the Dempsey Challenge goes to provide free assistance services to cancer patients.

Stuart Arbuckle, vice president and general manager of Amgen Oncology, said that biomedical firm that develops cancer medicines will be the principal sponsor of the 2012 Dempsey Challenge, as it has been this year and the two past years.

Sunday’s cycling rides will leave the park beginning at 7:30 a.m.

LEWISTON — “It’s a great day to be alive,” shouted Laura Davis as she stood on a park bench and addressed several hundred cancer survivors who were preparing to walk in the Dempsey Challenge on Saturday morning.

Davis, who is the 2011 Amgen Breakaway from Cancer Survivor Award-winner, urged people to share their stories. “There’s a story behind every bib on all the people out there,” who are participating in the weekend of walking, running and cycling events based at Simard-Payne Memorial Park.

One of those stories was behind the bib bearing the 5K walk number of Steve Dunwoody of Falmouth. He had just completed the route in a wheelchair.

“This is going to be an annual thing for me,” Dunwoody said. It was his second year in the Dempsey Challenge 5K. He said he made the ride in honor of an uncle and grandparents who died of cancer. His effort also was inspired by a Navy fighter pilot friend who was a cancer victim.

A broken back in a 1999 ski accident is the reason for his use of a wheelchair, Dunwoody said.

Linda Porter of Norway, who did both the 10K run/walk and the Survivor Walk, is both a caregiver for her husband’s bout with cancer and a survivor of breast cancer.

She said the Dempsey Challenge “is an awesome thing for Lewiston. I have been to some programs at the Dempsey Center and they are just so supportive of people going through such a hard time. It’s just amazing.”

Jaye Sadler walked in memory of her sister, Diane Harvey, who participated in last year’s Survivor Walk but died of lung cancer in recent months. Sadler walked with a group of friends in colorful tie-dyed T-shirts labeled “Harvey Wallbangers.”

Not all of the Dempsey Challenge stories were about cancer. Some people wanted to share memories of TV star Patrick Dempsey as a young boy in his hometowns of Buckfield and Turner.

One of Dempsey’s early schoolteachers was Bill Van Tassel, now a journalist and photographer for area publications. He said he remembers stories of Patrick and a school friend entering a vacant factory building in Turner. The boys made their way to an upper floor where one somehow broke a window, Van Tassel said, and Patrick, knowing it was wrong for them to go there, hoped his father wouldn’t find out.

A current resident of Buckfield (who did not want to give her name) remembered when she and her friends and neighbors would see a kid who was always juggling and attracting attention with his lively personality.

She said people would always ask, “Who is that kid, anyway?”

She recalled that Maine vaudevillians Denise and Benny Reehl took Dempsey under their wing and encouraged his performing talents.

Everyone now knows who that kid is. He earned the nickname of Dr. McDreamy in his role as Dr. Derek Shepherd on TV’s “Grey’s Anatomy,” and he starred in feature films, including “Enchanted,” “Made of Honor,” and “Can’t Buy Me Love.” He founded the Dempsey Center at Central Maine Medical Center in 2008 in response to a need for a cancer patient support facility, following his mother’s successful fight against ovarian cancer.

At the main stage late Saturday morning, Dempsey told the Festival in the Park audience, “Go around, have a good time and celebrate life. That’s what we are here to do.”