The first MTR train on the new South Island Line left Admiralty station for South Horizons at 5.55am on Wednesday.
MTR enthusiasts gathered as the corporation’s Operations Director Adi Lau delivered a welcoming speech on the concourse shortly before departure.
The event was live-streamed on the MTR Facebook page.
The opening of the new line means that all 18 districts of the city are now linked by metro.
The MTR predicts some 170,000 people will be served each day by the new line.
The new stations include Ocean Park, Wong Chuk Hang, Lei Tung and South Horizons, with an estimated 11 minutes of travel time between Admiralty and South Horizons.
The three stations do not have customer service centres – instead, electronic video service points and roving staff members will assist passengers.
Admiralty station – known for its huge traffic flow – has now doubled in size to accommodate the new line.
Correction 11:25am: A previous version of this article stated that the line was the first to use driver-less trains. In fact, the Disneyland line also uses driver-less trains.