Factories have been closed to ensure blue skies, potential troublemakers detained and a quarter of the residents have left: welcome to Hangzhou, a city China's ruling Communist Party is determined will look its best for the G20 summit.

From fines for hanging out laundry to restrictions on rice, Beijing has gone to extremes to ensure participants in the global pow-wow will leave with a glowing impression of their host nation.

Few summit-watchers expect policy fireworks when leaders from the world's biggest economies gather in the city for talks aimed at breathing life into still-sluggish growth.

But China's image-sensitive rulers see the meeting as a chance to showcase their country's emergence as a global powerhouse.

And they are taking no chances.

More than two million of the city's nine million citizens are expected to leave, state media say, many enticed by free trips offered by government-run tour firms and taking advantage of a week-long paid vacation employers have been ordered to give them.

Many of the hundreds of thousands of migrant workers who live hand-to-mouth from construction jobs or working for small businesses have already departed.

"Most migrants have gone home. If you don't close, you could be fined or even arrested," said a man surnamed Hou, who runs a small shop that is now shuttered, along with most nearby restaurants.

In an effort to reduce the smog that often chokes eastern China, restrictions extend far beyond the city.

Factories within a 300-kilometre (185 mile) radius of the city -- the heartland of China's textile industry -- have been forced to shut down for 12 days.

Even in Shanghai -- some 200 kilometres away -- 255 factories have been shuttered.

"Many plants like us are closed, losing a lot of money," a woman in her 50s who works for a sock manufacturer in the nearby city of Datang told AFP, asking not to be named.

"I heard the meeting is only for two days. This is all for China's image?"

-Dirty laundry-

Described as the "most beautiful and elegant city in the world" by 13th-century traveller Marco Polo, Hangzhou's island-dotted West Lake has been celebrated by Chinese artists for centuries.

Now the area is under tight police control, with uniformed officers on every street and rigorous security checks involving x-rays for anyone seeking to come close to the water.

"Police are checking IDs on subways, buses, and on streets. There are helicopters patrolling the sky, buzzing all day long. It's like we are about to go to war," a worker at a local internet company told AFP.

Others were enjoying the quiet. "It's quite nice to drive with so few cars on the street," said student Shao Tianyu.

Posters urging locals to "support the G20" and be a "civilised Hangzhou resident" dot the streets. Inside apartment compounds, notices announce bans on setting-off fireworks and flying remote-controlled drones.

"It is forbidden to hang clothes to dry on balconies within this compound" one poster said, threatening a fine of up to 1000 yuan ($150).

"Its because foreigners are coming," explained a security guard surnamed Wu.

The preparations are even having an impact on dinner tables.

After a top official declared last month that "not one single grain of unsafe rice" would enter Hangzhou for the summit, the staple saw big price rises, one local grain supplier said.

- 'Limiting my freedom' -

Representatives from several large European charities will be staying away after failing to secure accreditation.

"Without accreditation it would be just sightseeing," said Gerd Leipold of environmental campaign group Climate Transparency.

Hotels have also tightened security, with some refusing guests from Xinjiang, home to the mostly Muslim Uighur minority, members of which have been blamed for deadly attacks in Chinese cities in recent years.

"We do not entertain clients from Xinjiang, Hong Kong or Taiwan," a receptionist at the Jinjiang hotel told AFP. "After G20 we can host them".

In other countries the summit has been a flashpoint for protesters hoping to bring their cause to a global audience. But China has detained dozens of dissidents to prevent any unrest, rights groups say.

"I know of more than 100 cases of people being detained," due to the summit, said Huang Qi, who runs a website monitoring grass roots protest in China.

Li Huachen, who hopes to draw attention to the alleged forced demolition of his home, said he had been ordered to travel during the summit.

"They call it a vacation, but really its limiting my freedom," he told AFP by telephone. "They are scared I will go to Hangzhou".