Police in Beijing have launched a crackdown on rogue security firms, some of which detain petitioners in "black jails", state media said Friday, but a rights group said the illegal practice was unlikely to end.

More than 130 firms employ some 300,000 security guards in the Chinese capital, but nearly half of these operate without a licence, the official Global Times reported.

"The city's security market is in complete chaos," the report quoted Zhang Bing, deputy director at the Beijing Public Security Bureau, as saying.

"Some companies have been operating outside the law... and a few have allowed their security guards to impede the flow of government work and to participate in illegal activities such as detaining people and setting up black jails."

According to the report, police have already busted two security companies contracted out by provincial governments to arrest petitioners coming to Beijing and detain them in "black jails."

These "jails" are often makeshift hotels or guest houses, and rights groups say there is no way of knowing how many there are in Beijing.

The report said a security guard at one company was even beaten to death for trying to persuade his boss to end the illegal activity.

Since ancient times, Chinese people have been able to bring their local grievances to Beijing to petition higher authorities for redress -- a practice carried forward by the ruling Communist Party.

But over the last decade, as the number of petitions grows, provincial officials have increasingly tried to stop petitioners from reaching higher authorities by detaining them first.

In 2010, the Hong Kong-based Chinese Human Rights Defenders (CHRD) documented that more than 2,600 petitioners were placed in "black jails" in Beijing.

But the activist group estimates the actual number is far larger.

Renee Xia, director of CHRD, said the six-month crackdown was unlikely to stop the violations, saying others such as policemen would likely step in to do the rogue security guards' job.

"It has (also) been the case that police from the provinces come into Beijing to "deport" the petitioners back to their own cities and detain them in local "black jails," she said.

Strikers clash with police in Shanghai: rights group
Shanghai (AFP) Dec 2, 2011 - More than 1,000 workers at a plant in China's commercial hub Shanghai went on strike for at least two days, some clashing with police, to protest staff being laid off, a rights group said Friday.

Several workers were injured in conflicts with police at the factory, owned by a Singapore electronics firm that supplies companies including Apple and computer maker Hewlett Packard, US-based China Labor Watch said in a statement.

The protest -- the latest in a spate of unrest in China as an increasingly demanding workforce faces off with employers struggling with high costs and falling exports -- broke out Wednesday after the company laid off about 1,000 people.

Staff claimed they lost their jobs without notice and were given inadequate compensation, China Labor Watch said. The workers were laid off because the company planned to move production elsewhere.

The strike appeared to be continuing Friday, with more than 50 workers wearing blue uniform jackets standing inside the factory as police in two vehicles looked on.

The factory's owner, Singapore-based Hi-P International, told AFP the impact of the strike was "very minimal" and said it was "working with the relevant authorities."

Shanghai police could not be reached for comment Friday. A police statement Thursday put the number of strikers at over 100 and said they blocked the gate of the factory -- which makes home appliances -- and disrupted production.

The factory's lease would end in the first half of next year and workers were unhappy with severance pay, it said.

Hi-P International manufactures for the telecommunications, consumer electronics and computing industries, according to its website.

The unrest comes as China's exports and manufacturing activity weaken, hit by falling demand due to economic woes in Europe and the United States -- both crucial markets for the export-driven economy.

Last month, more than 7,000 workers went on strike at a factory in the southern province of Guangdong making New Balance, Adidas and Nike shoes, clashing with police in a protest over layoffs and wage cuts.

Also last month, hundreds of female workers walked off the job at a bra factory in the southern city of Shenzhen, the manufacturing metropolis that borders Hong Kong, to demand overtime payments.