Pilots, firefighters, nurses and ambulance workers who battled through the bombing raids of the Blitz came together at St Paul's Cathedral to commemorate the ordeal.
A service at St Paul's marked the 70th anniversary of the start of the raids.
Thousands of people across the UK were killed and injured in the raids by German forces in 1940 and 1941.
The Dean of St Paul's described the cathedral as "a national icon of defiance and hope amidst conflict".
The City of London Salute celebrated those who worked to protect Britons.
The Dean, the Right Reverend Graeme Knowles, told some 2,500 people who had gathered for the ceremony: "As we stand in this building, which itself stood as a national icon of defiance and hope amidst conflict and uncertainty, we pray for all those who hold memories of a campaign which protected this island from invasion.
"We give thanks for the bravery and service of the members of the Royal Air Force as well as those who supported them from the land, especially for those who gave of their time to protect the life and heritage of our homes and cities.
"We recall too the cost of the campaign, commending to God those whose lives were taken in service and innocence in the struggle for power and freedom."
Blitz, the German word for "lightning", was applied by the British press to the tempest of heavy and frequent bombing raids carried out over Britain.
This concentrated direct bombing on industrial targets and civilian centres, with heavy raids on London.
The date of 7 September has been chosen for the commemoration as it is exactly 70 years to the day since the German offensive switched to non-military targets, with nearly 1,000 German planes crossing the English Channel to attack cities.
The scale of the attack rapidly escalated. In September alone, the German air force dropped 5,300 tonnes of high explosives on the capital in just 24 nights.
In their efforts to "soften up" the British population and to destroy morale before the planned invasion, German planes extended their targets to include the major coastal ports and centres of production and supply.
More than 400 people were killed on the first day alone. Hundreds more were injured and huge fires burned across London.
On the steps of St Paul's Cathedral, Lord Mayor of the City of London Nick Anstee explained what he felt the significance of the service was to London and the UK as a whole.
He told the BBC: "I believe that services like these are a commemoration and a recognition of the contribution our forebears made.
"Only last Saturday, when I went to Duxford, I saw a wave of 16 Spitfires take off from Duxford and the crowd went silent in absolute total respect."
The RAF's principal chaplain, the Venerable Ray Pentland, who gave the sermon, said everyone who played a role in the Battle of Britain, from Bomber Command to those who provided refreshments, was part of the story which changed the course of history.
"Without the Battle of Britain there would have been no D-Day, no victory in Europe," he said.
London was not the only city targeted by the Luftwaffe.
In Coventry, on 14 November 1940, 500 German bombers dropped 500 tonnes of explosives and nearly 900 incendiary bombs in 10 hours of unrelenting bombardment, a tactic later emulated on an even greater scale by the RAF in its attacks on German cities.
Most industrialised areas were targeted, not least of all Merseyside, said to be the most heavily bombed region outside London. More than 4,000 people were killed, many during the worst raids in Christmas 1940 and May 1941.
In Belfast, about 900 people were killed in one night and about 100,000 - a quarter of the city's population - lost their homes.
Doris Nundy lived in Leyton in east London and kept a diary through the Blitz. After she died, her daughter Alison found it. Alison read an extract for the BBC.
She said: "Last night, from six o'clock until after 11 o'clock, the air was full of the drone of enemy planes and the pounding of guns.
"Soon after the siren was sounded, a blaze was reflected in the sky towards the city, and as the evening wore on, this reflection broadened until it seemed the whole sky must be on fire."
The commemoration event was organised jointly by the Lord Mayor of the City of London and the RAF Association.
It was intended to recognise the effort of everyone directly involved in the Battle of Britain and the Blitz.
Those who attended included people who worked as firefighters, nurses, ambulance workers, as well as those who were children at the time, Battle of Britain pilots and other military personnel.
Organisers encouraged Londoners and tourists to turn out to support World War II veterans and current military personnel, for a march-past through the City after the service.
WWII Spitfire, Lancaster and Dakota aircraft flew overhead at the same time.
The main air offensive against British cities diminished after May 1941, with the change of direction of the German war machine towards Russia.
However, sporadic and lethal raids, using increasingly large bombs, continued for some time.