Shanghai Expo is China's new showcase to the world
By Chris Hogg
BBC News, Shanghai
Highlights from the opening fireworks
The World Expo is opening in the Chinese city of Shanghai - the first time a developing country has hosted the event.
Chinese state media says the country has spent much more on the event than was spent on the Beijing Olympics.
At least 70 million visitors - almost all of them Chinese - are expected to visit pavilions and exhibitions staged by more than 240 countries and organisations from around the world.
But it is hard to say for certain these days what a World Expo is for.
In the past they were used to promote a country's strength as a world power.
The first one - the Great Exhibition at Crystal Palace in London in 1851 - was designed to demonstrate Britain's industrial, military and economic superiority.
To China, this year's Shanghai World Expo is no less important.
It is an opportunity, officials say, to promote China's "soft power" - to show off its growing influence in the world - and, of course, for others to try to win favour with the Chinese.
The national pavilions, some of them extraordinary structures, are evidence of the lengths some nations are prepared to go to try to deepen their ties with the hosts.
Lyndall Sachs says countries need to be represented at the Expo
"Imagine the message it would send to China if we weren't here," says the diplomat in charge of Australia's pavilion, Commissioner General Lyndall Sachs. "We need to make sure we continue to grow this relationship".
But, in an age where you can exchange information instantaneously at the click of a computer mouse, is there still a need to spend billions of dollars staging such a fair?
Expo spokesman Xu Wei says there is. "It is a big platform, a big event for cultural exchanges," he says.
"We are bringing the whole world - different countries, different peoples - to this area, so it's a good opportunity for Chinese people to have 'face to face' contact with international society."
But outside the site, others complain the authorities are actively trying to restrict such contacts.
Feng Zhenghu is the activist who gained worldwide attention earlier this year after he camped out in Tokyo's Narita Airport to protest at China's refusal to allow him to return home to Shanghai.
Now back in the city, he complains that in recent days he has in effect been confined to his apartment.
Mr Feng says the local authorities want the Expo to pass off smoothly
When I visited him, officials parked outside checked my credentials and only after they have established I am a foreign journalist, allowed me inside.
He says other visitors have been turned away.
The police seized four of his computers in the middle of the night last week and detained him briefly.
He had been trying to set up an alternative online Expo - highlighting miscarriages of justice.
"The Shanghai authorities are very nervous about their Expo," he says.
"They've promised it will go ahead without a hitch. They're under a lot of pressure. That's why they've taken extreme measures against people like me."
Security is tight in the Expo Park and across the city.
China's leaders have said this is the most important event to be staged in the country this year.
Shanghai has around 46,000 police officers. State media says more than 8,000 others have been drafted in to help from elsewhere, and a further 1,000 brought out of retirement.
At the Expo site during trial runs ahead of the event's opening, the People's Liberation Army was helping to martial the long queues outside the most popular pavilions.
Soldiers were also stationed outside the entrances to metro stations across the city.
Shanghai newspapers have reported that 6,000 people were arrested in the city as patrols were stepped up ahead of the event.
Those selling fake DVDs and other illegal goods have been forced to adopt a low profile.
For months, Shanghai has felt like a big building site as workers raced to complete infrastructure for the Expo.
The city now has a new airport terminal, new metro lines, roads, parks and public areas.
This will be one important legacy of the event. Some say the infrastructure investment will create economic benefits for Shanghai long after the six-month Expo is over.
Paul French, a Shanghai-based analyst for the consultancy Access Asia, is not convinced though.
"Hotels tell me they are getting a bit of a bounce in May, but after the third week their occupancy rates are slumping again," he says.
"I don't know anyone who's saying, 'Shanghai's having an Expo, let's buy a house!' Perhaps they're just talking up their stock."
The site itself is huge, twice the size of Monaco, straddling both sides of the Huangpu river in the city centre.
Many of the structures have won praise from the early visitors.
Britain's "seed cathedral" is another crowd pleaser
The British 'seed cathedral' (also dubbed 'the dandelion' or 'the hedgehog' because of the spikes it is made of) is a crowd-pleaser, as is the Spanish pavilion, made of rattan and woven wicker.
In some pavilions, it is what is not on display that is most interesting. North Korea's did not appear to contain any images of the "dear leader" Kim Jong-il during the "soft opening" of the park to visitors last week.
And in the China pavilion, a huge red structure that towers over the rest of the park, some of the main attractions were accompanied by commentary in Chinese only.
Perhaps that reflects the main purpose of this event. To show the millions of Chinese that will visit the Expo, just how important and influential their country has become.
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