There was a minute's silence as the stone monument honouring those who died was revealed at the Titanic Memorial Garden on Sunday morning.
It carries the names of the 1,512 passengers and crew who died in the 1912 disaster, carved into five bronze plaques - the first time all the victims have been recorded together.
Jack Martin, a 12-year-old great, great nephew of the ship's assistant surgeon Dr John Simpson, helped Belfast Lord Mayor Niall Ó Donnghaile with the unveiling.
After laying a wreath, Jack said: "I'm glad I have this connection with the Titanic and I'm very glad I have the chance to commemorate all of those who died."
Descendants of those who passed away when the Belfast-built ship sank also laid floral tributes to the victims beside the memorial.
John Martin, who is Dr Simpson's great nephew, said: "It is very emotional, more so than I thought it would be and I am grateful to be here.
"It is a wonderful way to commemorate all of those who died."
The focus of the world is on Belfast and we are doing her proud. We are all proud of this ship - what happened was a disaster, she was not.
Una Reilly, chairwoman of the Belfast Titanic Society
The service in Belfast featured music from singer Brian Kennedy and reflections from actor Dan Gordon and minister the Rev Ian Gilpin.
Rev Gilpin said: "We behold the Titanic memorial, we remember all those who perished and whose names are herein inscribed - men, women and children who loved and we loved, their loss still poignantly felt by their descendants.
"In the permanence of granite, marble and stone may there be a permanence in our remembrance, in diversity, in the colour and fragrance of the flowers of the memorial, that the memorial be an acknowledgement of the diversity of humankind."
The nine-metre wide memorial records the name all of those who died on board Titanic after she hit an iceberg in the North Atlantic.
Many existing memorials have failed to include the Titanic crew or musicians. On this one there is no distinction between first-class passengers and others, with names in alphabetical order.
"It is significant that this is the only monument that lists alphabetically all the 1,512 souls that were lost on the Titanic," Mayor Niall Ó Donnghaile told UTV.
"Historically they have been remembered as a collective or, worse, based on their class status, but we have remembered all of them in Belfast and it is great the families are here."
The service at City Hall ended with a minute's silence for private reflection before the hymn Nearer My God to Thee, which it is claimed was played by the band before the ship broke in two and sank, rang out.
Earlier, those on board a cruise ship retracing the route the RMS Titanic took on her maiden voyage laid wreaths at the spot where she disappeared underwater.
The captain of the MS Balmoral made an announcement when the clocks reached 11.40pm on Saturday, the time of the collision 100 years ago, and the names of the dead were read aloud.
Around 50 of the 1,309 passengers have a direct family link to Titanic, and say they want to complete the journey from Southampton to New York their relatives never made.
"I shall feel a sense of accomplishment that I have achieved what I set out to do," said 81-year-old Patricia Watts, whose grandfather George MacKie was a second-class steward.
"I think the service will be a very memorable occasion, slightly sad, but also for a lot of people it will be the event of the cruise."
The service in Belfast came at the end of three weeks of events to mark the building, voyage and sinking of the Titanic 100 years ago.