The 2010 World Expo has opened in the city of Shanghai in what China hopes will be further proof of its rising global influence.

Almost 250 countries and international organisations are showcasing their culture in an event themed around sustainable development.

Many are doing so in pavilions with radical architecture.

World leaders, including France's President Sarkozy, are attending the lavish opening ceremony.

"Expo 2010 Shanghai is now open!" Chinese President Hu Jintao declared during the gala opening, in which 2,300 performers and musicians from all over the world took part.

They were celebrating the Expo slogan: "Better city, better life in music and dance".

Tens of thousands of fireworks and lasers then lit up the city's riverfront - in what organisers promised would be the biggest-ever multimedia event, the BBC's Chris Hogg in Shanghai says.

Some 70 million visitors - mostly Chinese - are expected to visit the Expo, which will be open for the next six months.

Chinese local media report that the cost of staging the event could be as much as $58bn (£38bn) - more than was spent on the Beijing Olympics in 2008. Some estimates say this figure could eventually be even higher.

Shanghai has increased security measures drastically in preparation for the event.

An additional 8,000 police officers have been brought in to help Shanghai's 46,000-strong police force to patrol the city, Chinese state media says.

'Oppressive' security

Residents living near the Expo site have complained about oppressive security measures.

"It's just not convenient to get in and out any more," Dong, a local resident, was quoted as saying by the Associated Press news agency.

Markets have been closed down to build Expo car parks, he added.

Six people who protested about having their homes destroyed to make room for the Expo have been sent to labour camps, the Hong-Kong based Chinese Human Rights Defenders were quoted as saying by AP.

Last week police seized four computers belonging to activist Feng Zhenghu, who had been trying to set up an alternative online Expo, highlighting alleged miscarriages of justice, our correspondent says.

The Expo is seen as an opportunity for countries to try to win favour with the Chinese, he adds.

Speaking at a press conference in Beijing on Thursday, President Sarkozy - whose relationship with China has been testy after he criticized a Chinese security crackdown in Tibet in 2008 - said France and China would "think and work together".