But what was initially meant to be a public event was turned into a more private showing for potential buyers as organisers feared a stampede of visitors, after the bowl drew crowds in Shanghai on Wednesday and Thursday.

"An object has rarely generated so much excitement and for security reason we thought it would be preferable for our clients to view it within the confines of a private room," Nicolas Chow, deputy chairman for Sotheby's Asia, told AFP.

The flower-shaped Ru dish from the Northern Song Dynasty (1086-1125) features six sharp notches on the edge and a subtly translucent matte glaze, and is believed to be the only one of its type in the world, Sotheby's auction house has said.

Chow has called "the superb Northern Song lobed washer from the fabled Ru kilns" ... the "star of the season".

"It is the first time in 30 years Sotheby's has auctioned off a piece of ceramic this rare. I would say this is about the highest degree of rarity you can find."

Of the 79 surviving Ru ware dishes, the "Ruyao Washer" is the only one that features an organic floral shape and an opaque glaze, he said.

It was expected to sell for up HK$80 million ($10.3 million) at Sotheby's spring auction in the southern Chinese city on April 4.

Other pieces to go under the hammer include a large, early 15th-century blue and white dish from the Ming Dynasty (1368-1644), and an Anhua stembowl from the Xuande reign (1426-35) featuring dancing dragon motifs.

More than 380 lots with an estimated worth of more than HK$650 million will be sold at the auction, Sotheby's said.

Hong Kong has emerged as one of the biggest auction centres after New York and London.

Chinese art prices have rocketed in recent years, fuelled by China's economic boom and a steady demand from rich Asian collectors, especially mainland Chinese buyers.

But a Sotheby's auction of imperial Chinese porcelain this time last year sold for less than pre-sale estimates.

After Shanghai and Beijing the bowl is to be shown in Taipei.

"The Taiwanese are probably among the most sophisticated collectors in the field of Chinese art," said Chow. "They'll pick something extraordinary to raise their collections."

Long jail terms, enforced disappearances and torture of dissidents amounted to a "downward spiral" in China's record, Hong Kong-based Chinese Human Rights Defenders (CHRD) said in its annual report.

The study found more than 3,800 cases of arbitrary detention last year, as well as over 100 cases of individuals tortured specifically because of their rights activism.

"(The crackdown) marked yet another low point in the downward spiral of China's human rights records, making 2011 the most repressive year since the rights defence movement began in the early 2000s," said Renee Xia, CHRD's international director, using a term coined by Chinese activists.

The group said it was particularly alarmed by the "widespread use of extralegal detention and enforced disappearance".

"The crackdown impacted not only the individual activists, but also menacingly conveyed a warning to the ordinary Chinese citizens: anyone who challenges the government will be punished," it said.

CHRD said one of the most alarming developments last year was the use of "enforced" disappearances, in which at least two dozen activists were taken by authorities and held for long periods of time in secret locations.

The internationally renowned artist Ai Weiwei was among several dissidents to be taken to unknown locations -- known as "black jails" -- and held for months without charge during the crackdown on government critics last year.

Other high-profile cases of enforced disappearance included three well-known rights lawyers, Teng Biao, Jiang Tianyong and Tang Jitian, who were held for several months before being released.

All three were previously treated with relative tolerance and CHRD said their lengthy detentions showed the heightened crackdown last year.

China had planned to make it legal to detain criminal suspects for up to six months in secret locations as part of changes to the country's criminal law, which is being debated by lawmakers ahead of a vote expected next week.

Proposed amendments to the law included a clause that allowed police to hold people suspected of terrorism or endangering national security in secret locations without notifying their families.

But on Thursday it emerged that China has abandoned those controversial plans following a public outcry.

Other activists were given unusually long jail sentences last year for subversion -- a charge rights groups say is often used to jail government critics.

Longtime dissidents Chen Wei and Chen Xi were imprisoned for nine and 10 years respectively at the end of December for subversion.

Rights groups said both men had signed Charter 08, a bold manifesto for democracy co-authored by Liu Xiaobo, the 2010 Nobel Peace Prize winner who was jailed for 11 years for subversion on Christmas Day 2009.

"Lengthy prison sentences handed out in 2011, like the use of enforced disappearance against high-profile activists, appear to be intended to normalise what has previously been rare or exceptional," said CHRD.

Moves by authorities to make hundreds of millions of Internet users use their real names when registering for weibos -- microblogs similar to Twitter -- was another disturbing development aimed at silencing government critics, it said.

"This measure is probably one of the most effective yet in reining in the power of microblogs to expose rights abuses and put pressure on the authorities," it said.