Shadowy China X-Men stoke social strains: report HONG KONG (Reuters) - Unchecked abuses in cities by shadowy para-police units derided by many Chinese as "arbitrary and thuggish" are intensifying social strains and undermining stability, international advocacy group Human Rights Watch said in a report on Wednesday. The report said China's "chengguan" para-police agencies were an often overlooked yet malign and less supervised component of China's public security apparatus, their action often directed at disadvantaged sectors of society. It said the agencies' influence had grown in recent years and stoked resentment at the grassroots level. The chengguan -- or Urban Management Law Enforcement units -- typically augment police in tackling lower-level urban crime. The report said they tend to be poorly supervised and lack a solid legal framework to check and govern their powers, resulting in a reputation for excessive force and impunity. "The chengguan have become synonymous among some Chinese citizens with arbitrary and thuggish behavior," the report said, citing interviews with 25 victims of chengguan abuses in six cities. "They're really sort of this black hole in terms of monitoring, surveillance and discipline," said Phelim Kine, the author of the report. The chengguan have been called "the epitome of the evils of public power" and derided as law-breaking "X-Men" in Chinese state media and microblogging sites. While the scale of chengguan operations is difficult to gauge accurately, Human Rights Watch said there were at least 6,000 chengguan personnel in Beijing alone, with easily hundreds of thousands spread across the country's urban centers. The chengguan have no legal authority to detain suspects and no regulatory framework yet exists to lay out the permissible scope of their duties or to investigate complaints and abuses. Many victims of para-police activity were migrant street vendors at the lower rungs of society who had been beaten, had their goods confiscated or were illegally detained or evicted. This is a substrata of the population which is taking, to a large extent, the brunt of the brutalities ... They're really at the sharp end of the chengguan spear," said Kine. As the Communist Party leadership gears up for a leadership transition after the upheavals linked to the dismissal of Chongqing party boss Bo Xilai, the abuses of the chengguan have done little to uphold the social stability the party craves. In 2011, the southern province of Guangdong saw riots in the city of Zengcheng, outside the booming provincial capital of Guangzhou, after chengguan personnel manhandled a pregnant migrant street vendor. Migrant workers rampaged and set fire to police stations and cars over several days. "We have been seeing increasingly more violence and increasingly larger scale public protests and public reactions to perceived chengguan malfeasance and malpractice," Kine said. Human Rights Watch urged China's leaders to crack down on chengguan use of excessive force and to develop mechanisms like new laws to prevent abuses and punish perpetrators. Reporting by James Pomfret; Editing by Ron Popeski