Pan Am 103, which was bound for New York, exploded less than an hour after takeoff from London on Dec. 21, 1988.
Many of the victims were American college students flying home for Christmas.
One man — former Libyan intelligence official Abdel Baset al-Megrahi — was convicted of the bombing. He was given a life sentence, but Scottish authorities released him on humanitarian grounds in 2009 when he was diagnosed with prostate cancer. He died in Tripoli last year.
Officials including Scottish leader Alex Salmond and relatives of some of the victims gathered at Lockerbie's Dryfesdale Cemetery, laying down wreaths and cards as bagpipes played in the background.
"In my heart, to me this is home and there was no other place I felt I should be on this very sad and special occasion," said Jane Schultz, who lost her 20-year-old son, Thomas. "It's nice and peaceful and it's where Thomas was, so it's like coming home."
Services will be held later Saturday both at London's Westminster Abbey and a local church in Lockerbie.
In the United States, U.S. Attorney General Eric Holder and other officials will speak at a ceremony at Virginia's Arlington National Cemetery. A service is also planned at Syracuse University in New York state, which lost 35 students returning from studies in Europe.
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