Similar to previous corruption cases involving high-level government officials, allegations against Mr Xu, as detailed in the Hong Kong media and mainland chat rooms, had him taking tens of millions of yuan in bribes. And as no corruption case involving senior officials is complete without the involvement of pretty young women, Mr Xu is alleged to have helped young mainland actresses migrate to Hong Kong in return for sex. This allegation has forced a number of actresses who recently obtained residency papers in Hong Kong to issue denial statements.

But one allegation against Mr Xu is particularly worth noting. Once a car technician, he was seen as climbing up the power ladder really fast, considering that he was not known for having strong backers in the central government. According to rumours among Shenzhen officials, he spent at least 20 million yuan (HK$22.7 million) to bribe superior officials to secure the mayor's position.

Rumour has it that he had raised even more money in an attempt to become the party secretary of Shenzhen, the most powerful job in the city. If the allegation proves true, this appears to be the first time a high-ranking government official is known to have bought a top position.

More importantly, this has brought to light the most hideous part of political corruption on the mainland - the buying and selling of government posts and official titles.

Rarely touched upon in either the official or overseas media, this phenomenon is rampant throughout the mainland hierarchy, from civilian jobs to military positions - officials bribe their superiors to get promotions and sell titles to subordinates.

The practice appears to have existed ever since Chinese civilisation began. Historians generally believe the systematic buying and selling of government posts began more than 2,000 years ago during the reign of Qin Shihuang , who is credited with unifying China for the first time.

The emperor decreed that farmers who donated a certain amount of millet would be awarded ranks of nobility. Since then, history is replete with details of emperors selling official titles to raise money for the coffers and mandarins selling them for personal gain. The practice was wiped out in 1949, when the Chinese Communist Party, which celebrates its 88th anniversary on July 1, took power. Party members were promoted mainly because of their loyalty to the party and then chairman Mao Zedong , as well as the fact that they had superiors as their backers.

But the phenomenon crept back not long after economic reforms began. It became rampant, particularly in the past two decades as the mainland's economy went on an unprecedented boom that has created lucrative opportunities for government and party officials.

In 2006, Xinhua quoted a former head of the party's organisation department responsible for official promotions and demotions as saying that buying and selling government posts and official titles had become one of the main forms of corruption on the mainland.

State media has occasionally carried reports of mid-level officials being sacked and arrested for trading government posts. Earlier this month, Xinhua reported that Zhou Guangquan, former party secretary of Chaohu, Anhui, stood trial for accepting bribes worth 4.1 million yuan from more than 30 people, including about 20 officials who were promoted after bribing him.

It goes without saying that an official who buys a position usually recoups and makes more money by selling positions to his subordinates, thus contributing to a systematic problem. Analysts say this kind of 'power trading' is common at all government levels, although it is probably more blatant at the local levels than in the central government.

In Mr Xu's case, he may not even have had to use his own ill-gotten gains to bribe others for a more powerful promotion. A high-ranking official can usually count on a group of entrepreneurs who will put up the money needed, expecting to be awarded lucrative contracts or other favours after the official is promoted.

According to mainland analysts who are knowledgeable about the issue, the institution where the buying and selling of posts and titles is the most rampant and kept under closest wraps is the People's Liberation Army. Ask any PLA soldier or officer and he is likely to confirm that one needs to use bribery for everything - from getting enlisted to promotions. Some demobilised officers said there were even generally acceptable and clearly marked prices for every rank from the squad leader upwards.