The typhoon Wipha was moving northwest across the sea north of Taiwan at about 25 kilometers per hour (16 mph) and was forecast to make landfall south of Shanghai early Wednesday morning, weather reports said.
The storm, which was whipping up waves up to 10 meters (36 feet) high, was upgraded from a tropical storm on Monday afternoon. With wind gusts of up to 300 kilometers per hour (190 mph), local meteorological officials said it could be the most destructive storm to hit the Shanghai area in years.
"The typhoon is very likely to develop into the worst one in recent years. We are still observing it. It's hard to say at this moment," said a man who answered the phone at the city's meteorological bureau. As is common with Chinese officials, the man identified himself only by his surname, Fu.
Shanghai and the coastal provinces of Zhejiang and Fujian to the south issued typhoon warnings requiring all vessels to return to shore or change course to avoid the storm, the official Xinhua News Agency said.
State-run newspapers reported that some 200,000 people living in coastal or low-lying rural areas of Shanghai were being evacuated as a precaution.
Zhejiang's flood control headquarters also ordered authorities to prepare for evacuations, patrol reservoirs and to be on the lookout for flash floods and landslides, Xinhua said.
It said nearly 30,000 fishing boats in the province had taken shelter in port by late Monday and ferry service with outlying islands had been suspended.
The deadliest storm to hit the China coast in recent years was Typhoon Winnie in 1997, which killed 236 people. Typhoon Rananim, with winds of more than 160 kph (100 mph), was the strongest typhoon to hit the Chinese mainland since 1956, killing nearly 200 people.
The choice of the city of Helsinki is not incidental as the capital of Finland had hosted US-Soviet negotiations on the limitation of nuclear stockpiles in 1969