Services are being held in Britain and Argentina to mark the 30th anniversary of the start of the Falklands War.
Some 255 British and 650 Argentine troops died after the UK sent a task force to the islands in response to Argentina's invasion on 2 April 1982.
The anniversary comes amid renewed tension, as Argentina has reasserted its claim to the archipelago.
The UK's prime minister has suggested the day is used to remember both the Argentine and British dead.
In a statement, David Cameron has also said that he remains committed to upholding British sovereignty over the islands.
Britain has controlled the Falklands since 1833 but Argentina claims the territory - which it calls the Malvinas - saying it inherited its rights to them from Spain.
British veterans of the war - and relatives of those who died - have paid their respects at Britain's National Memorial Arboretum in Staffordshire.
There a single candle was lit as a gesture to mark the anniversary. It will remain alight for 74 days - the length of the conflict.
In his statement, Mr Cameron said: "30 years ago today the people of the Falkland Islands suffered an act of aggression that sought to rob them of their freedom and their way of life.
"Today is a day for commemoration and reflection: a day to remember all those who lost their lives in the conflict - the members of our armed forces, as well as the Argentinian personnel who died."
Mr Cameron saluted the "heroism" of the soldiers, sailors, airmen and marines who took part in the operation which freed the islanders from Argentine rule.
He said "We are rightly proud of the role Britain played in righting a profound wrong. And the people of the Falkland Islands can be justly proud of the prosperous and secure future they have built for their islands since 1982.
"Britain remains staunchly committed to upholding the right of the Falkland Islanders, and of the Falkland Islanders alone, to determine their own future.
"That was the fundamental principle that was at stake 30 years ago: and that is the principle which we solemnly reaffirm today."
Argentina's President Cristina Fernandez de Kirchner is expected to visit the southern port of Ushuaia on Monday, to remember the Argentine troops that died.
President Fernandez is expected to lead rallies to commemorate the Argentine dead and light an eternal flame.
Prior to her arrival, Argentine veterans held a vigil for the fallen.
British Falklands War veteran Simon Weston, who was badly injured when his ship the Sir Galahad was attacked, told the BBC he did not believe the Argentina's military capability was as strong a threat as it was at the time of the initial conflict.
He added he felt Britain's stance at the time was the right one, adding: "I always believe that people's independence, people's freedom, people's right to self-determination is always worth defending. You've only got to go the islands and meet the islanders and understand what it means to them."
Argentine veteran Alejandro Meringer said he felt he "lost his youth" because of the war. He said he experienced great joy when he was told by an officer he would be helping to "recover Las Islas Malvinas" but this was replaced by sadness when their mission was unsuccessful.
Argentina has asked for negotiations about sovereignty but the British government says it will not discuss the issue without the agreement of the islanders.
London has accused Buenos Aires of trying to impose an economic blockade on the islanders, after it banned Falklands-flagged ships from docking in its ports, as well as those of other countries which are members of the Mercosur trading block.
Argentina has complained about what it calls British "militarisation" in the south Atlantic, after it was announced one of the Royal Navy's newest warships was to be deployed to the region.
BBC World affairs editor John Simpson said while a new armed conflict remains unlikely, Argentina was now using diplomatic weapons to push its claim over the Falklands.
The defeat of the Argentine forces led directly to the collapse of the military dictatorship led by Gen Leopoldo Galtieri, who was later jailed in Buenos Aires for "incompetence" during the war.
The British prime minister at the time was Margaret Thatcher but she is not expected to play a part in the commemoration of the 30th anniversary due to ill-health.