The convicted Lockerbie bomber has been flown home to Libya after being freed from a Scottish prison on compassionate grounds.
Abdelbaset Ali al-Megrahi, 57, was jailed in 2001 for the atrocity which claimed 270 lives in 1988.
The decision to release Megrahi, who has terminal prostate cancer, was made by the Scottish Government.
US president Barack Obama said the decision was "a mistake" and some US victims' families reacted angrily.
Some 189 Americans were among those who died in the explosion.
A police convoy left Greenock Prison, where Megrahi was serving his sentence, just an hour after the announcement of his release was made.
It was greeting by angry jeers from a small group of local residents.
Megrahi was taken to Glasgow Airport where he boarded an Afriqiyah Airways Airbus plane bound for Tripoli, wearing a white track suit and clutching his prison release papers.
The aircraft took off shortly before 1530 BST and arrived in the Libyan capital shortly after 1930 BST.
The government said it had consulted widely before Scottish Justice Secretary Kenny MacAskill made his decision on applications for Megrahi's compassionate release or his transfer to a Libyan jail. He told a media conference on Thursday that he had rejected the application for a prisoner transfer.
However, after taking medical advice it was expected that three months was a "reasonable estimate" of the time Megrahi had left to live.
He ruled out the option of the Libyan being allowed to live in Scotland on security grounds.
And Mr MacAskill stressed that he accepted the conviction and sentence which had been handed to Megrahi.
"Mr al-Megrahi did not show his victims any comfort or compassion. They were not allowed to return to the bosom of their families to see out their lives, let alone their dying days. No compassion was shown by him to them," he said.
"But that alone is not a reason for us to deny compassion to him and his family in his final days."
Mr MacAskill continued: "Our justice system demands that judgement be imposed, but compassion be available.
"For these reasons and these reasons alone, it is my decision that Mr Mr Abdelbaset Ali Mohmed Al-Megrahi, convicted in 2001 for the Lockerbie bombing, now terminally ill with prostate cancer, be released on compassionate grounds and be allowed to return to Libya to die."
Mr MacAskill had been under intense pressure from the US government to keep Megrahi behind bars, with US Secretary of State Hillary Clinton saying his release would be "absolutely wrong".
"Compassion and mercy are about upholding the beliefs the we seek to live by, remaining true to our values as a people - no matter the severity of the provocation or the atrocity perpetrated," he added.
In a statement released after his departure from HMP Greenock, Megrahi continued to protest his innocence.
He said: "The remaining days of my life are being lived under the shadow of the wrongness of my conviction.
"I have been faced with an appalling choice: to risk dying in prison in the hope that my name is cleared posthumously or to return home still carrying the weight of the guilty verdict, which will never now be lifted.
"The choice which I made is a matter of sorrow, disappointment and anger, which I fear I will never overcome."
Reacting to the decision, US president Barack Obama said: "We have been in contact with the Scottish Government, indicating that we objected to this and we thought it was a mistake."
He said they had also contacted the Libyan government to ask that Megrahi not be "welcomed back" but instead placed under house arrest.
"We've also obviously been in contact with the families of the Pan Am victims and indicated to them that we don't think this was appropriate," he added.
The Libyan government has played down claims Megrahi would return to Tripoli a hero.
He will be required to live permanently at a given address in Libya, must agree any change of address and must not travel from Libya without consent, the Scottish Government said.
The families of American victims of the Lockerbie bombing reacted angrily to the news of his release.
Kara Weipz, of Mt Laurel, New Jersey, who lost her 20-year-old brother Richard Monetti, said: "I don't understand how the Scots can show compassion. It is an utter insult and utterly disgusting.
"It is horrible. I don't show compassion for someone who showed no remorse."
New York state resident Paul Halsch, whose 31-year-old wife was killed, said of Mr MacAskill's decision: "I'm totally against it. He murdered 270 people.
"This might sound crude or blunt, but I want him returned from Scotland the same way my wife Lorraine was and that would be in a box."
However, British relatives' spokesman Dr Jim Swire, who lost his daughter Flora in the atrocity, said he believed Megrahi had "nothing to do with" the bombing.
"I don't believe for a moment that this man was involved in the way that he was found to have been involved," he said.
"I feel despondent that the west and Scotland didn't have the guts to allow this man's second appeal to continue because I am convinced had they done so it would have overturned the verdict against him.
"It's a blow to those of us who seek the truth but it is not an ending. I think it is a splitting of the ways."
The BBC's Christian Fraser in Tripoli said that until now, Libyan officials had been careful not to comment in case they jeopardised the release, wary of last minute interventions by the US Secretary of State Hillary Clinton.
Officially there are unlikely to be any triumphant statements, but given the personal involvement of Mr Gaddafi it will no doubt be seen as further evidence of his growing stature on the international stage, our correspondent said.
It is rumoured that he has asked to see Megrahi when he returns, and the timing is perfect - in 12 days' time Libya celebrates the 40th anniversary of the revolution that brought Muammar Gaddafi to power, he added.
Our correspondent said Megrahi's release has been billed by the leader as the new dawn for Libya, and to many it will be viewed as a more palatable ending to one of the darkest chapters in the country's history.
Megrahi was convicted of murder in January 2001 at a trial held under Scottish law in the Netherlands.
A first appeal against that verdict was rejected the following year.
However, in 2007 the Scottish Criminal Cases Review Commission granted him a second appeal.
It subsequently emerged he was suffering from terminal cancer but a bid to have him granted bail was refused.
His second appeal got under way this year but shortly afterwards applications were made for both his transfer to a Libyan jail and release on compassionate grounds.
Earlier this week the High Court in Edinburgh allowed Megrahi's application to drop his second appeal.