The third and final module is scheduled to dock in October, and Tiangong — which should have a lifespan of at least 10 years — is expected to become fully operational by the end of 2022.
China has launched the second of three modules to its permanent space station, in one of the final missions needed to complete the orbiting outpost by year's end.
A live feed on state broadcaster CCTV on Sunday showed the 23-tonne Wentian ("Quest for the Heavens") laboratory module launching on the back of China's most powerful rocket, the Long March 5B.
The launch took place at 2:22 pm (0622 GMT) from the Wenchang Space Launch Center on the southern island of Hainan.
Space agency staff, seen on the live feed observing the progress from a control room, cheered and applauded when the Wentian separated from the rocket about 10 minutes after the launch.
The launch was "a complete success", CCTV reported shortly after.
China began constructing the space station in April 2021 with the launch of the Tianhe module, the main living quarters, in the first of 11 crewed and uncrewed missions in the undertaking.
The Wentian lab module, 17.9 metres long, will be where astronauts can carry out scientific experiments, along with the other lab module yet to be launched - Mengtian ("Dreaming of the Heavens").
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Beijing's space programme
Wentian features an airlock cabin that is to be the main exit-entry point for extravehicular activities when the station is completed.
It will also serve as short-term living quarters for astronauts during crew rotations on the station, designed for long-term accommodation of just three astronauts.
Mengtian is expected to be launched in October and, like Wentian, is to dock with Tianhe, forming a T-shaped structure.
The completion of the structure, about a fifth of the International Space Station (ISS) by mass, is a source of pride among ordinary Chinese people and will cap President Xi Jinping's 10 years as leader of China's ruling Communist Party.
On board the space station are Shenzhou-14 mission commander Chen Dong and team mates Liu Yang and Cai Xuzhe.
They are slated to return to Earth in December with the arrival of the Shenzhou-15 crew.
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