HONG KONG (Reuters) - Macau, the world's largest gambling hub, arrested five police officers over graft and cooperation with illegal gangs, in the first crackdown on its own law enforcement officials, as authorities vowed to clean up the Chinese casino enclave.

Thursday's arrests come as the former Portuguese colony targeted corruption and abuse of power in the past year, arresting nine officials for graft, a stark contrast with 2014, when no high-profile officials were questioned.

Macau, one of China's special administrative regions, has been battered as a prolonged anti-corruption campaign and slowing economic growth have pushed revenues to lows not seen in more than five years. Gambling revenue tumbled 34 per cent in 2015, in a second consecutive annual fall.

Six people, including a retired police officer, were arrested for bribery and making illicit profits of 1.8 million patacas (S$323,077).

"Secretary for Security Wong Sio Chak said he felt very sad, and highly concerned about the incident," the government said in a statement published online on Friday (Jan 15).

Police would follow up with an in-depth investigation and strengthen mangement throughout the force, it added.

Just two years ago, Macau, the only place in China where citizens can legally gamble in casinos, was booming, as revenues outstripped Las Vegas by more than seven times and shares in the Hong Kong-listed gaming companies soared in value.

While the tiny territory still makes revenues five times those of Vegas, it faces challenges from weaker demand to gamble in its 37 casinos and tighter rules dimming its appeal for high roller VIP players to drop millions, as they did in the past.

Casino operators including Sands China, Wynn Macau , MGM China, Galaxy Entertainment, Melco Crown Entertainment and SJM Holdings, are all building new multi-billion dollar properties on a stretch of reclaimed land called Cotai, which is modelled after Las Vegas' neon strip.