Thousands of people, including 2,000 re-enactors dressed in costume, are marking the 950th anniversary of the Battle of Hastings.
Battle Abbey, in East Sussex, widely accepted as the site of one of the most famous events in British history, is the focus for a weekend of events.
The battle, between William of Normandy and Anglo-Saxon King Harold II, ended in the latter's death.
A group of "warriors" recreating the 300-mile march made by King Harold to the battle site arrived during the morning.
They left York last month, travelling on foot and horseback and living in Saxon-style camps along the way.
Re-enactor Nigel Amos said: "In 300 miles we have seen some great countryside, met an awful lot of fantastic people and engaged with school groups.
"And we have got a feeling of what it would have felt like to have come that distance on that fateful march.
"This is a tribute to King Harold and all of the warriors of 1066 because they are what made this country."
King Harold defeated a Viking army at the Battle of Stamford Bridge in East Yorkshire, on 25 September, 1066, before travelling south to face the Normans.
On 14 October, he fought and was killed at the Battle of Hastings.
Duke William then seized the English throne.
Roy Porter, from English Heritage, described the final battle as a "cataclysmic" event.
"It changed English society, changed the way in which English politics worked, changes in architecture, the introduction of castles, the language we speak today is a result of 1066, that mixture of Old English and French," he said.
Home Secretary and local MP Amber Rudd, along with the mayor of Falaise in Normandy, Eric Mace, will be among dignitaries from both sides of the English Channel marking the anniversary.
The annual re-enactment of the battle, taking place at Battle Abbey on Saturday and Sunday, is a sell-out English Heritage said.
Meanwhile, a bust of William the Conqueror was unveiled at Berkhamsted Castle in Hertfordshire, where English leaders surrendered to the king in 1066.
Thousands of soldiers were killed on 14 October 1066.
The dead will be commemorated at a service later at Battle Church in Hastings.
Happy Birthday, Norman Britain.
Those Saxons are an obstreperous lot . . . i mean, you Saxons.