By John Simpson
BBC World Affairs Editor, Chongqing
The municipality of Chongqing claims to have the biggest population on earth.
At the last count, in 2005, Chongqing had well over 30 million inhabitants with permits to live there, more than entire countries like Canada or Poland. No-one knows how many people live there illegally.
It has grown to this vast size in only 13 years. Before 1997, Chongqing was just another of China's middle-ranking provincial cities.
But the sudden development has meant big money, and big money tends to bring big corruption. In Chongqing, the corruption went right to the top.
Senior officials could be bought and sold. Organised crime moved in.
The city is in the midst of a major clampdown against racketeering and corruption. Among the 3,000 people arrested so far are around 50 officials, including the chief of Chongqing's justice department, Wen Qiang.
He is related by marriage to Xie Caiping, the woman who ran Chongqing's biggest illegal gambling casino, right across the road from the city's Supreme Court building.
One of his main associates, a former judge, committed suicide in prison.
The man behind the crackdown is a national politician, Chongqing's Communist Party secretary Bo Xilai.
Mr Bo, tough, sophisticated and ambitious, comes from the Communist aristocracy - his father was one of Mao Zedong's leading comrades.
Many see Bo Xilai as a potential future national leader
Yet Mr Bo is very much an example of the new China - he dresses superbly and sent his son to Britain to be educated at Harrow - one of the most famous and expensive private schools - and latterly at Oxford University.
In his efforts to stamp out corruption in Chongqing, Mr Bo has attracted accusations that he uses some highly questionable methods.
Zhu Mingyong, a Beijing lawyer who represents one of the leading accused - now sentenced to death - told the BBC he had not been allowed proper access to his client.
He also claimed that his client had been tortured.
Mr Zhu said he felt intimidated by the arrest and imprisonment of another defence lawyer involved in the case.
This man was accused of fabricating evidence when he represented the main mafia boss in Chongqing, and was sentenced to 18 months.
The corruption sweep has netted some 3,000 people so far
But Bo Xi Lai knows that his campaign against corruption is hugely popular and he is said in the Western press to have ambitions to run the country.
He sends out text messages at random to ordinary people in Chongqing, much as Barack Obama did when he was running for the American presidency.
People we spoke to in the street mostly liked the idea.
Corruption is the number one concern in China.
Because of his campaign, one Western China-watcher suggested recently that if the National People's Congress were able to choose the country's political leadership, Mr Bo would romp home next year, when the final decision is made.
As it is, only a small group at the very top of the Communist Party is involved in making the selection: and they seem nervous of Mr Bo's charisma.
Things are changing very fast in China and they seem to be changing with particular speed in Chongqing.
People are prepared to speak out in a way they would never have dared to in the past.
Just a few years ago, a lawyer like Mr Zhu, defence counsel for the mafia boss who has been sentenced to death, would never have gone on the BBC and accused the authorities of torturing his client.
The pace of change in Chongqing is fast partly because the city authorities know they have to work hard to get international attention and investment.
It is far inland, and has few of the natural advantages enjoyed by the big cities of China's eastern seaboard.
As a result, it feels the need to be more open and welcoming.
Some of the old nervousness remains, though. My team and I were invited to Chongqing by the city's government. I had been asked by the British consulate there to give a lecture about the power and responsibility of the media.
A hundred or more senior journalists accepted invitations. Not one of them actually turned up. Their places were taken by local media students.
Mr Bo, whom I have met more than once, was too busy to see us. The mayor of Chongqing, who had fixed a time for us to interview him, had to leave town unexpectedly at the last moment.
And everywhere we drove, we were followed discreetly by the same silver Honda, presumably from the security police.
Even in China's newest, biggest, most go-ahead city, the old habits still die hard.
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