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Another 113 people were wounded, the official Xinhua news agency said, revising down an earlier higher figure. It said the attack had taken place late on Saturday evening.
"It was an organised, premeditated violent terrorist attack," Xinhua said.
Police shot dead five of the unidentified attackers and were searching for around five others, it said.
Kunming resident Yang Haifei told Xinhua that he was buying a ticket when he saw a group of people, mostly wearing black, rush into the station and start attacking bystanders.
"I saw a person come straight at me with a long knife and I ran away with everyone," he said, adding that the attackers caught those who were slower. "They just fell on the ground."
Graphic pictures on the Twitter-like microblogging service Sina Weibo showed bodies covered in blood lying on the ground at the station.
There was no immediate word on who was responsible.
Chinese President Xi Jinping ordered no effort be spared to track down those behind the attack.
"Severely punish in accordance with the law the violent terrorists and resolutely crack down on those who have been swollen with arrogance," Xinhua quoted him as saying.
"Understand the serious and complex nation of combating terrorism," Xi said. "Go all out to maintain social stability."
Domestic security chief Meng Jianzhu was on his way to the scene, Xinhua said.
Weibo users took to the service to describe details of what happened, though many of those posts were quickly deleted by government censors, especially those that described the attackers, two of whom were identified by some as women.
Others condemned the attack.
"No matter who, for whatever reason, or of what race, chose somewhere so crowded as a train station, and made innocent people their target - they are evil and they should go to hell," wrote one user.
The attack comes at a sensitive time as China gears up for the annual meeting of parliament, which opens in Beijing on Wednesday and is normally accompanied by a tightening of security across the country.
China has blamed similar incidents in the past on Islamist militants operating in the restive far western region of Xinjiang, though such attacks have generally been limited to Xinjiang itself.
China says its first major suicide attack, in Beijing's Tiananmen Square in October, involved militants from Xinjiang, home to the Muslim Uighur people, many of whom chafe at Chinese restrictions on their culture and religion.
Hu Xijin, editor of the influential Global Times newspaper, published by the ruling Communist Party's official People's Daily, wrote on his Weibo feed that the government should say who it suspected of the attack as soon as possible.
"If it was Xinjiang separatists, it needs to be announced promptly, as hearsay should not be allowed to fill the vacuum," Hu wrote.