"Thick smoke is spewing from the site, and several dozen floors are burning ferociously," state broadcaster CCTV reported.
"Firefighters have begun work to extinguish the flames and conduct rescues at the scene," it added.
The blaze consumed a tall building that housed an office of state-owned telecommunications company China Telecom, the report said.
A photograph released by CCTV showed orange flames searing through the building in a built-up area of the city, as black smoke billowed into the sky.
A video posted on social media by a local news outlet appeared to show the outside of the tower had been charred black.
The China Telecom said in a statement on social media, “by around 4:30 pm today, the fire at our No. 2 Communications Tower in Changsha has been extinguished. No casualties have yet been discovered and communications have not been cut off."
The 218-metre (715-foot) building was completed in 2000 and is located near a major ring road, according to CCTV.
Changsha, the capital of Hunan province, has a population of about 10 million people.