All boats in the area around the Harland and Wolff shipyards, where the pride of the White Star fleet was built, then sounded their horns.
In 1911, thousands of cheering well-wishers gathered at the same place to celebrate the historic moment.
A century on, the mood was again one of celebration at the event on the Queen’s Island slipway which focused more on the ship’s construction than its fate.
After the flare was fired, crowds clapped for exactly 62 seconds, the length of time it took for the liner to roll down the slipway in 1911.
The Titanic sank on her maiden transatlantic voyage 11 months after her launch, with the loss of more than 1,500 lives, when she struck an iceberg.
Among the invited guests at the commemoration were schoolchildren and representatives from the four other cities and towns directly connected to the Titanic story: Cherbourg in France, Cobh in Co Cork, Liverpool and Southampton.
The Harlandic and Queen’s Victoria male voice choirs sang a number of hymns during the half-hour service close to the almost-complete £100 million (€115m) Titanic visitors centre, which is set to open ahead of next year’s centenary of the liner’s sinking in 1912.
Earlier, a major exhibition on the Titanic opened at the nearby Ulster Folk and Transport Museum, boasting some artefacts recovered from the liner that have never been put on public display before.
Descendants of many of the men who helped build the ship, some of whom sailed on the first voyage and died in the maritime disaster, attended the service.
Lord Mayor of Belfast, councillor Niall O’Donnghaile, said Belfast’s role in the Titanic story had been overlooked in the past.
“The Titanic story is probably one of the most fascinating, amazing, poignant, thought-provoking and absorbing tales from the last century, if not the last millennium,” he said.
“For too long, Belfast’s part in the Titanic story, and the role of the people of Belfast in bringing Titanic to life, has been neglected.”