A little after 1900 GMT the exact anniversary of the atrocity will be remembered.
One of the commemorations was held in the Garden of Remembrance, which has been built on the area where the plane came down in Lockerbie.
The US consul in Edinburgh, Lisa Vickers, attended the ceremony.
She said: "There were 180 people that were American citizens on board that flight, 35 of them students at Syracuse University and we still remember very much the events of that night 20 years ago."
Up to 300 people, including relatives and some former employees of the Pan Am airline were attending a private memorial at Heathrow Airport chapel led by the Rev John Mosey, whose daughter Helga died in the disaster.
Local priest explains his Lockerbie role
For many who lived through it, the memories remain fresh despite the time which has passed.
George Stobbs, Lockerbie's police inspector at the time, recalls the events of 20 years ago with great clarity.
"Nobody actually knew what had happened, we realised an aircraft had come down but I thought it was a military aircraft," he said.
"Once I got into Sherwood Crescent I could see flames along the roadside, the footpaths were burst and there were gas pipes fractured - there were dancing flames coming up from them.
"Hedges were on fire, drop pipes on the side of houses were on fire and they were in turn climbing up and setting fire to the roofs."
It was only later in the evening that the scale of the death toll at Lockerbie began to emerge.
The town has moved on in the intervening years, but Mr Kerr said it was right that it should mark a major anniversary.
The discovery of the nose cone revealed the full scale of the disaster
Eyewitness Maxwell Kerr said: "You realise just how lucky we are to have had another 20 years of life."
"These people who died - that was it snuffed out in one second. I think you have never to forget this, it is too big an event, it is too big a disaster."
Paying tribute, First Minister Alex Salmond said: "I know that through the events being organised in Lockerbie, at Syracuse University, and at other locations in the UK and the US, that fitting tribute will be paid to those who so tragically lost their lives and those, in south of Scotland and beyond, whose lives have been affected by the atrocity."
He added "I offer my support to all involved in marking the anniversary and, in particular, my condolences to those who will be mourning the loss of a loved one."
Jim Murphy, Secretary of State for Scotland, said: "Our eternal sympathy goes out to all those who lost family members and friends and who to this day feel the dreadful effects of that terrible event."
Former Libyan intelligence officer Abdelbaset Ali Mohmed Al Megrahi was found guilty of mass murder following a trial at Camp Zeist in the Netherlands in 2001.
Megrahi, who was recently diagnosed with terminal prostate cancer, has consistently denied responsibility for the bombing and a second appeal against his conviction will be heard by the courts next year.
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