Harley's four-day event was expected to draw 200,000 to 300,000 people. The festivities center on Milwaukee's lakefront and include a parade of 10,000 Harleys through the city.
Gene and Sherrie Cocanower, from Lafayette, Ind., were among hundreds of people waiting for gates to open at the celebration's official opening Thursday morning, watching a spectacle of Harley models rumble past.
"You got every walk of life here," ranging from CEOs to those who own only their bike and a backpack, said Gene Cocanower.
"You don't know who you're standing next to and it really doesn't matter as long as they don't smell."
Many were biking to the home of Harley and others were flying in, but as CBS News reports, some were arriving by ferry.
The SS Badger carried bikers from Ludington, Michigan to Manitowoc, Wisconsin, shaving four hours off their trip to Milwaukee.
People waved from freeway bridges Wednesday to welcome riders streaming in from across the nation. Several accidents involving motorcycles snarled freeway traffic at times.
"I'm loving it. I'm here. I made it through Chicago," said Rhonda Rackliffe, who rode some 1,800 miles from Minot, Maine.
She said she started riding her own Harley a year ago after spending years on the back of her husband's bike.
"I was bored riding in the back," she said.
This time, Rhonda and Randy Rackliffe rode separate bikes, making the trip with their 20-year-old son, Josh, "to visit the best family in the world."
"We're all brothers and sisters," Randy Rackliffe said.
Dave and Rita Chester from Philadelphia stopped at a truck stop on their way and encountered 11 people from England, also riding Harleys to the celebration.
"It's just like a fraternity, a massive club, where everyone has one commonality, a Harley-Davidson motorcycle," he said.
The grand finale Sunday will be a 100th birthday party with music and fireworks at the Lake Michigan shore.
Classic rock acts such as Kansas, Peter Frampton and REO Speedwagon will perform, and 80,000 square feet of exhibits will showcase antique motorcycles, clothing, film clips and bikes owned by Elvis Presley and Jon Bon Jovi.
On Wednesday, Harley provided an early start for many riders by marking the first 20 years of HOG, or Harley Owners Group.
Up to 100,000 motorcyclists were expected to cruise through the annual HOG rally at the Washington County Fair Park at West Bend, 30 miles north of Milwaukee, by the time it ends Saturday night.
The company started the club in 1983 as a way to allow people to share the Harley experience.
About 30,000 riders joined in its first year, and the group now numbers about 750,000 in more than 1,300 chapters worldwide. Members meet regularly for rides, parties, fund-raisers, motorcycle seminars and other events.
Frank Lamantia of New York City said he rode a motorcycle as a young man but gave up his bike in 1988 when he had a son. He got another motorcycle in 2001, and hooked up with HOG to find people to ride with.
"Now it's a form of recreation for me," he said. "You can ride around or you can ride with a group, but you're also an individual so you make your bike individual" through the selection of accessories.