By Clare Jim and Yimou Lee
HONG KONG, Oct 4 (Reuters) - More than a thousand rival protesters, some wearing helmets, faced off in a densely populated, gritty district of Hong Kong on Saturday, fueling concerns that the Chinese-controlled city's worst unrest in decades could take a more violent turn.
After a night of trouble which resulted in 19 arrests, supporters of the city's pro-Beijing government rallied next to pro-democracy protesters in Mong Kok, a working class neighborhood near the popular shopping district of Tsim Tsa Shui.
Many Hong Kong residents expressed anger and frustration at police handling of the unrest, with some accusing security forces of co-operating with criminal gangs, failing to make arrests and helping some attackers to exit the scene quickly.
"We condemn the violence used against Hong Kong civilians yesterday," said student leader Joshua Wong.
"I find it ironic how people accuse us of being violent and radical and now after one week of peaceful protests the ones who use violence is them - the government that allows Triads to exercise brutality on peaceful protesters."
After a week of largely peaceful demonstrations demanding Beijing grant Hong Kong the unfettered right to choose its own leader, the mood turned ugly on Friday night in an area notorious for being the home of Triads.
A rowdy crowd of around 2,000 filled the narrow streets of Mong Kok, one of the world's most densely populated areas, in the small hours of Saturday and the atmosphere was highly charged as police in riot gear tried to keep them under control.
Among those detained by police were eight suspected gang members. Eighteen people were injured, including six police officers, according to local broadcaster RTHK.
Student activists, established protest groups and ordinary Hong Kongers have joined forces to present Beijing with one of its biggest political challenges since it violently crushed pro-democracy protests in and around Tiananmen Square in 1989.
Tens of thousands of protesters have staged sit-ins across Hong Kong over the past week, demanding the city's pro-Beijing leader Leung Chun-ying step down and China reverse a decision made in August to handpick the candidates for Hong Kong's 2017 leadership election.
After police fired tear gas against mostly student protesters last weekend, the demonstrations have been largely peaceful.
But on Saturday, some pro-democracy supporters - umbrellas in hand and wearing motor-bike helmets, gloves and black leather jackets - braced for trouble. Scores of yellow signs around the site occupied by pro-democracy supporters read: "Police and mob working together - an alternative violent crackdown."
The pro-Beijing group, Caring Hong Kong Power, that organized the Mong Kok rally on Saturday afternoon said it supported the use of guns by police, if necessary, and also the deployment of the People's Liberation Army (PLA).
Hong Kong leader Leung has said the use of PLA soldiers would not be necessary.
One of the main student groups behind the "Occupy Central" protest movement said it would pull out of planned talks with the Hong Kong government, because it believed authorities had colluded in the attacks on demonstrators in Mong Kok.
Secretary for Security Lai Tung-kwok said allegations police were co-operating with the Triads were false.
The notorious gangs operate bars, nightclubs and massage parlors across Mong Kok, an area of high-rise apartment blocks across the harbor from the main protest areas.
At times over the past week, police have left the streets, saying they wanted to ease tensions, though the reason for their apparent absence from this scene on Saturday morning was unclear.
Police have defended their handling of fighting in the area, saying they had exercised "dignity and restraint and tried our best to keep the situation under control."
But Amnesty International issued a statement criticizing them for "(failing) in their duty to protect hundreds of peaceful pro-democracy protesters from attacks by counter demonstrators."
The Foreign Correspondents' Club in Hong Kong, the Hong Kong Journalists' Association (HKJA) and local broadcaster RTHK all strongly condemned violent attacks on members of the press during street clashes over the past 24 hours.
"Hong Kong is in a turmoil unseen after the 1967 riot. Without an effective monitor of the media, the condition will only deteriorate further, making any rational discussion impossible," the HKJA said in a statement.
About 1,000 protesters maintained their blockade outside administrative buildings in the city center.
PROTESTS "BUT A DAYDREAM"
The ruling Communist Party's official People's Daily, in a front page editorial on Saturday, praised Hong Kong police for their restraint in the face of what it said was lawless protests, including "poking" of police with umbrellas.
The protests will never spill over into the rest of China, the newspaper added. "For the minority of people who want to foment a 'color revolution' on the mainland by way of Hong Kong, this is but a daydream."
Facing separatist unrest in far-flung and resource-rich Tibet and Xinjiang, Beijing is standing firm on Hong Kong, fearful that calls for democracy there could spread to the mainland, especially if successful.
Demonstrations across Hong Kong have ebbed and flowed since last Sunday, when police used pepper spray, tear gas and batons to break them up in the worst unrest in Hong Kong since the former British colony was handed back to Chinese rule in 1997.
At times, tens of thousands of people gathered to block roads and buildings in central areas of the global financial center, bringing them to a virtual standstill.
China rules Hong Kong through a "one country, two systems" formula underpinned by the Basic Law, which accords Hong Kong some autonomy and freedoms not enjoyed on the mainland and has universal suffrage as an eventual goal.
But Beijing decreed on Aug. 31 it would vet candidates who want to run for chief executive at an election in 2017, angering democracy activists, who took to the streets. (Additional reporting by Twinnie Siu, Elzio Barreto, Charlie Zhu, Clare Baldwin, Joseph Campbell, Donny Kwok, James Pomfret, Bobby Yip, Irene Jay Liu, Farah Master, Diana Chan, Kinling Lo, Venus Wu and Anne Marie Roantree in HONG KONG, and Ben Blanchard in BEIJING; Writing by Jeremy Laurence; Editing by Kim Coghill)