Thousands attended the early morning service at Hyde Park Corner and up to 400 people took part in a parade before the wreath-laying at the Cenotaph.
Anzac Day commemorates the first major battle involving Australian and New Zealand forces during World War One.
A service was also held at Westminster Abbey.
The national anthems of New Zealand and Australia were sung as the service ended.
The first time Anzac Day was commemorated in the capital was when King George V attended a Westminster Abbey service in 1916, a year after the Anzac (Australian and New Zealand Army Corps) landings at Gallipoli.
Since then, the services have become an important moment for thousands of expatriate and visiting New Zealanders and Australians, who honour the sacrifices of their countrymen and women in all wars.
Addressing the crowds at the dawn service, Alexander Downer, Australian high commissioner to the UK, who also laid a wreath, said: "When we reflect on Anzac Day we imagine the Gallipoli landings, what it must have been like, at dawn on the water, in sight of that rugged shoreline - and a collectively held breath, a leaden silence about to be broken.
"We consider the enthusiasm, the courage, and the heroism of the Anzac troops - ordinary men fighting for God, King and empire, for their mates, for adventure, for a world without war."
Members of veterans' associations, service and ex-service personal and their families took part in the parade at the Cenotaph.
Crowds, including Australia and New Zealand ex-pats, lined the streets to watch the ceremony which featured prayers read by schoolchildren and the national anthems of all three countries.
The eight-month campaign on the Gallipoli peninsula in Turkey was one of the bloodiest of the war and the allied operation was an attempt to force the Ottoman empire out of the war.
Prince Harry has laid wreaths during a dawn service at London's Wellington Arch and on behalf of the Queen at the Cenotaph to mark Anzac Day in the UK.