Hopper's simple wooden coffin was ushered into the adobe chapel at historic San Francisco de Asis church.
Hopper, who was twice nominated for Oscars and received a star this year on the Hollywood Walk of Fame, died Saturday at age 74 at his home in Los Angeles. He was diagnosed with prostate cancer in 2009.
Nicholson, cast alongside Hopper in "Easy Rider" as a drunken young lawyer in a breakout role, described Hopper as "an all-around guy."
"It was a very singular relationship I had with him, like nobody else. We were soul mates in a way. I really miss him," Nicholson said.
The church's bells chimed as Hopper's casket was brought into the chapel and again as it was taken out after the two-hour service. The rumbles of about half a dozen motorcycles could be heard in the distance as the casket was taken away in a funeral procession. A private burial followed.
Hopper first came to New Mexico in 1969 to scout locations for the iconic "Easy Rider." He lived in Taos for 12 years through the early 1980s.
Mourners who poured out of the chapel after the service described it as "lovely," "tender" and "beautiful," without mention of Hollywood or Hopper's public persona.
During the service, Hopper's son, Henry Lee Hopper, drew tears from many in the crowd as he read from Walt Whitman's collection of poems, "Leaves of Grass." Other relatives also spoke.
Many said the Mass gave them a sense of Hopper's spirit, and stories of his early days as a Kansas farm boy, his wild side and the time he spent in Taos making films and art continued to be shared after the memorial at a gathering of his friends and family.
"He was the spark plug of this town," said R.C. Israel, an art dealer and a longtime friend of Hopper.
Hopper has been called everything from an artist and an icon to a rebel, but Henry Lee Hopper said trying to sum up his father's life in a few words is nearly impossible. To the 19-year-old, Hopper represents an eternal flame of inspiration.
"You find something that you believe in and you really realize you've got to fight for it. Some people fight for a minute, but there are very, very few people in this world who fight from the day they're born to the day they go out. I would say Dennis was undoubtedly one of these people," the younger Hopper told The Associated Press.
Jina Brenneman, curator of the Harwood Museum of Art in Taos, where Hopper's work has been showcased, remembered Hopper as a fighter for the creative process.
"Every ounce of his being was focused on doing or promoting creativity and imagination and not letting any naysayer get in the way. He wasn't afraid of bucking the system," she said.
Lisa Law, a photographer and longtime friend of Hopper, remembered his keen photographic eye and his sense of adventure.
"There wasn't a minute that wasn't interesting being with him," she said. "He was very open to all sorts of different experiences, and that's what made him such a rich fellow. To have Dennis gone, it leaves a hole."
Before the service, Kilmer, who lives near Pecos in northern New Mexico, said the last conversation he had with Hopper was on an airplane flight, recalling that Hopper could always make him laugh.
Even Wednesday, there were plenty of smiles as Hopper was remembered.
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