Victims described attackers dressed in black bursting into Kunming station in the southwestern province of Yunnan and slashing indiscriminately as people queued to buy tickets, prompting shock and outrage.
Police shot dead at least four attackers and were hunting for the others, the official Xinhua news agency said.
The Kunming city government said the attack was orchestrated by separatists from the northwest region of Xinjiang, a vast and distant area home to the mostly-Muslim Uighur minority, Xinhua reported.
Xinjiang is periodically hit by violent clashes between locals and security forces but attacks targeting civilians are rare, and are unheard of in Yunnan, more than 1,600 kilometres (1,000 miles) from Xinjiang and a popular tourist destination.
The attack comes months after three members of the same family from Xinjiang crashed their car into crowds of tourists in Tiananmen Square in Beijing, the symbolic heart of the Chinese state, killing two, before setting it on fire and dying themselves, according to authorities.
A knife victim named Yang Haifei, who was wounded in the chest and back, told Xinhua that he had been buying a train ticket when the attackers approached.
"I saw a person come straight at me with a long knife and I ran away with everyone," he said, while others "simply fell on the ground".
- 'Stabbing whoever they saw' -
Some who managed to escape were desperately looking for missing loved ones.
"I can't find my husband, and his phone went unanswered," Yang Ziqing was quoted as saying.
She said she had been waiting for her train to Shanghai "when a knife-wielding man suddenly came at them".
Officers sealed off a wide area around the station, reports said, while police were questioning people at the site.
The attackers were dressed in similar black clothing, the official China News Service said, citing witnesses.
"A group of men carrying weapons burst into the train station plaza and the ticket hall, stabbing whoever they saw," it said.
State broadcaster CCTV called the incident a "terrorist attack" on its Weibo account.
President Xi Jinping called for "all-out efforts" in the investigation and for the attackers to be punished "in accordance with the law", Xinhua said.
UN Secretary General Ban Ki-moon condemned "in the strongest terms" the "terrible attack on civilians", his spokesman said in a statement, adding he "notes that there is no justification for the killing of innocent civilians and hopes that those responsible will be brought to justice".
Beijing's top security official Meng Jianzhu arrived in Kunming early Sunday and visited the scene of the attack, which came days before the opening of the annual session of the legislature, the National People's Congress, Xinhua reported.
Xi and Premier Li Keqiang sent condolences to the victims and their families.
- 'They will go to hell' -
Photos posted on Sina Weibo -- a Chinese version of Twitter -- showed blood spattered across the station floor and medical staff crouching over bodies lying on the ground, although the authenticity of the images could not be verified.
The pictures showed crowds gathered outside among police officers and ambulances. The injured had been delivered to hospitals around the city, local television station K6 said.
Pictures on the 163.com news portal also showed what it claimed was one of the attackers, who was lying on a stretcher surrounded by police.
The website also showed a local man near the train station carrying a large stick, which it said was for "self-defence".
Images of spectacles, shoes and baggage strewn across the floor of the waiting room behind police tape, were also posted online.
A Weibo user going by the name HuangY3xin-Dione who was at a restaurant near the scene told how she saw a group of men in black with two long knives chasing people, Xinhua reported.
Another eyewitness told the Beijing News that she had seen two women in black walking towards the station and that some of the attackers had their faces covered.
The incident was by far the most discussed topic on Weibo, where many users expressed their outrage at the attackers.
"Targeting ordinary people in a terrorist attack is disgraceful," said one user. "They have nothing to do with this issue."
"No matter who did this, for what purpose, and no matter which race, to target innocent people at a train station is an evil choice. Their hearts will be punished and they will go to hell," said Li Chengpeng, a social commentator and government critic who has more than seven million followers.
Incidents involving Uighurs are often labelled "terrorist attacks" by Chinese authorities, although the description is rarely used for incidents carried out by members of the Han majority seen as having grievances against society or the state.
Beijing maintains that unrest in Xinjiang is caused by terrorist groups seeking independence, an account denied by Uighur rights groups who complain of widespread religious repression and economic discrimination.