Waves several metres high hit the coastline as the storm made landfall in Zhejiang province, south of Shanghai.
The deaths were caused by a landslide triggered by the storm’s downpours in the municipality of Wenzhou, around 400 kilometres south of Shanghai, national television station CCTV reported.
“Torrential rains caused a landslide on a mountain that blocked a river below,” it said, adding that the resulting “dam” created a lake which swept downstream when it burst.
More than a million people were evacuated from their homes ahead of the storm, the official Xinhua news agency reported. Some 110,000 people were housed in shelters.
Over 250,000 residents in Shanghai and 800,000 in Zhejiang province had been evacuated due to the typhoon, and 2.72 million households in Zhejiang had power blackouts as strong wind and rain downed electricity transmission lines, state media reported.
Some 200 houses in six cities in Zhejiang had collapsed, and 66,300 hectares of farmland had been destroyed, CCTV said.
The monster storm made landfall in the early hours in Wenling City, packing winds of 187 kilometres per hour, and was expected to churn up the east coast towards Shanghai, Xinhua added.
Footage broadcast on CCTV showed a torrent of muddy water surging through the streets and emergency workers rescuing a man trapped under rubble and fallen trees.
One street in Yueqing city, part of Wenzhou, was shown completely flooded.
China issued a red alert as the storm approached on Friday, before downgrading the level to orange as winds eased on Saturday morning.
The storm was predicted to reach Jiangsu province by the early hours of Sunday and veer over the Yellow Sea before continuing north and making landfall again in Shandong province, CCTV said.
Coastal businesses in Zhejiang were shut and the Ministry of Emergency Management warned of potential risk of fire, explosions and toxic gas leaks at chemical parks and oil refineries.
Lekima has been downgraded from a super typhoon to a typhoon as it headed northwards inland.
More heavy rain was forecast for the Shanghai area and the neighbouring provinces of Anhui, Jiangsu and Zhejiang, with authorities warning of possible flash floods, mudslides and landslides caused by the downpours.
In Zhejiang province alone, nearly 300 flights were cancelled, and ferry and rail services were suspended as a precaution.
In Shanghai, the high-speed maglev train that links the city to one of its airports was suspended. Shanghai Disneyland was also closed for the first time since the amusement park opened in 2016.
Lekima had earlier swept past the northern tip of Taiwan on Friday, where nine people were injured, thousands of homes lost power temporarily and more than 500 flights were cancelled.
Last September, Typhoon Mangkhut slammed into mainland China where authorities evacuated more than two million people, after it left a trail of destruction in Hong Kong and Macau and killed at least 59 people in the northern Philippines.