Abdelbaset al-Megrahi, the only person convicted over the 1988 Lockerbie bombing above Scotland which killed 270 people, has died at his home in Libya.
Megrahi, 60, was convicted by a special court in the Netherlands in 2001.
He was freed from Scottish jail in 2009 on compassionate grounds because of cancer, stirring controversy when he outlived doctors' expectations.
UK Prime Minister David Cameron said it was a day to remember the 270 victims of "an appalling terrorist act".
Mr Cameron, who is in Chicago for a Nato summit, said Megrahi should never have been freed, Reuters news agency reports.
Scottish First Minister Alex Salmond also said Megrahi's death was an occasion to remember the victims of Lockerbie.
He said Lockerbie was still a live investigation and that Scottish prosecutors had never believed Megrahi was the only person responsible.
Megrahi's release sparked the fury of many of the relatives of the victims of the Lockerbie disaster. The US - whose citizens accounted for 189 of the dead - also criticised the move.
But others believed he was not guilty of the bombing.
Dr Jim Swire, whose daughter Flora died at Lockerbie, called Megrahi's death a "very sad event".
"Right up to the end he was determined, for his family's sake... [that] the verdict against him should be overturned," said Dr Swire, who is a member of the Justice for Megrahi group.
Died at home
His brother Abdulhakim said on Sunday that Megrahi's health had deteriorated quickly and he died at home in Tripoli.
He told the AFP news agency that Megrahi died at 13:00 local time (11:00 GMT).
Megrahi's sister told the Libyan Wal news agency that his funeral would take place at Tripoli's main cemetery on Monday, following early afternoon prayers.
Megrahi, a Libyan intelligence officer, always denied any responsibility for the bombing of Pan Am Flight 103 in December 1988.
It remains the deadliest terrorist incident ever to have taken place on British soil.
All 259 people aboard the plane, which was travelling from London to New York, were killed, along with 11 others on the ground.
Investigators tracing the origins of scraps of clothes wrapped around the bomb followed a trail to a shop in Malta which led them, eventually, to Megrahi.
He and another Libyan, Al Amin Khalifa Fhimah, were indicted by the Scottish and US courts in November 1991.
But Libya refused to extradite them. In 1999, after protracted negotiations, Libya handed the two men over for trial, under Scottish law but on neutral ground, the former US airbase at Camp Zeist in the Netherlands.
Their trial began in May 2000. Fhimah was acquitted of all charges, but Megrahi was found guilty and sentenced to a minimum of 27 years in prison.
He served the first part of his sentence at the maximum-security prison at Barlinnie, in Glasgow, but was transferred in 2005 to Greenock prison.
He lost his first appeal against conviction in 2002 but in 2007, his case was referred back to senior Scottish judges. He dropped that second case two days before he was released.
Last August, after the fall of Libyan leader Col Muammar Gaddafi, Megrahi was reported to be "in and out of a coma" at his home in Tripoli.
There have been calls for him to be returned to jail in the UK or tried in the US.
But shortly after they toppled Col Gaddafi, Libyan rebel leaders said they would not extradite Megrahi or any other Libyan.
The BBC's Scotland correspondent James Cook says Scottish and American officials have been to Tripoli, trying to persuade the new Libyan government to grant visas to detectives from Dumfriesshire.
They are still searching for the answers to the questions of who ordered the bombing and who else was involved, our correspondent says, but it is not clear whether the Libyans will co-operate.
However, a spokesman for the interim government in Tripoli, the National Transitional Council (NTC), told Reuters that that Megrahi's death would not end its investigations into Lockerbie.
"The Libyan government will continue to investigate the crimes committed by the Gaddafi regime using other witnesses," NTC spokesman Mohamed al-Harizy was quoted as saying.
Last September, it emerged that former UK Prime Minister Tony Blair had raised Megrahi's case in talks with Gaddafi in 2008 and 2009 in Libya, shortly before Megrahi was freed.
At the time, Libya was threatening to sever commercial links with Britain if Megrahi was not released.
But Mr Blair's spokesman told Col Gaddafi it was a case for the Scottish authorities and no business deals were discussed.
In his last interview, filmed in December 2011, Megrahi said: "I am an innocent man. I am about to die and I ask now to be left in peace with my family."
He had previously claimed he would release new information about the atrocity but little new has emerged.
Megrahi had rarely been seen since his return to Tripoli, but he was spotted on Libyan television at what appeared to be a pro-government rally in July 2011.