The famous mother and daughter, who died just a day apart in December, are being remembered at Forest Lawn Cemetery, where they were buried together earlier this year.
The service inside a 1,200-seat auditorium is also being shown on big screens at the cemetery and live-streamed on the internet.
Grace Farenbaugh from Burbank, California, was the first fan to arrive outside the Hall of Liberty at 5.30am.
"I'm a huge fan of both of them," she told the Press Association.
"I feel they are part of my family. I knew Carrie for a short while. She was helping me with a book. She was as funny and genuine as anyone can imagine."
Liza Rios-Proprofsky from Orange County, California, said she was a huge fan of Star Wars actress Fisher and she wanted to honour her with "like-minded people".
"Star Wars has been a part of our child narrative," she said. "When Carrie Fisher passed, it actually like a member of the family had left us.
"When I was a little girl, here was this strong woman who was a princess who could take charge.
"We knew that if we came here we would be with people who are like minded, that would understand how we felt."
Fisher, who shot to fame as Princess Leia in Star Wars, died aged 60 on December 27 after suffering a heart attack on a flight from London to Los Angeles.
A day later her mother, Singin' In The Rain star Reynolds, 84, died after a suspected stroke.
The memorial will feature a tribute song from Fisher's close friend James Blunt, which will be unveiled during a photo montage.
Her daughter Billie Lourd will attend but is not expected to make a speech, while Fisher's beloved dog Gary will also feature, TMZ said.
The Gay Men's Chorus of Los Angeles will perform along with dancers from the Debbie Reynolds Dance Studio.
Costumes and memorabilia from the careers of Reynolds and Fisher will also be on display.
The two actresses were laid to rest at a funeral on January 6, where the ashes of Fisher were carried in an urn shaped like a Prozac pill.
Fisher had spoken publicly about her battle with bipolar disorder and drug problems during her life.
Additional reporting from Press Association.