Ex-fighter pilots and relatives of war heroes joined commemorations as Sir Winston Churchill's stirring "so much owed by so many to so few" speech was read out, prompting tears in the crowd.
Actor Robert Hardy began reading out the speech at 3.52pm, exactly 70 years after the wartime Prime Minister delivered it in Parliament.
Among those gathered for the ceremony at the Churchill War Rooms were Dame Vera Lynn and Sir Winston's daughter, Lady Soames.
Speaking afterwards, Dame Vera, the singer of We'll Meet Again, said: "It brought it all back.
"So much was owed to so few - and it is wonderful that some of those brave men are here."
Lady Soames, 88, said: "It is very moving because 70 years ago I was in the House of Commons to hear my father deliver the speech.
"For me it has particular meaning but I find it wonderful that I look around this crowd and for all of us somehow the speech rang a bell."
The crowds waving Union flags cheered as the world's oldest Spitfire and a Hurricane fighter emerged over the trees to fly low over London's Government buildings, stirring memories of the battle which began on July 10 1940 and ended on October 31 that year.
More than 2,900 British, Commonwealth and Allied aircrew took part and successfully fought off the Luftwaffe.
The triumph helped wreck Hitler's plans to invade Britain and lay the foundations for Allied victory five years later.
It was celebrated in Churchill's speech of August 20 1940, when he told MPs: "Never in the field of human conflict was so much owed by so many to so few."