HONG KONG -- Macau police said Monday they arrested 150 people in raids at casinos and hotels across the Chinese territory following a spate of violence that raised fears of a revival of gang warfare in the world's most lucrative gambling market.

Police questioned 1,300 people in an exercise named "Operation Thunderbolt 2012" that was carried out with authorities in Hong Kong and mainland China. They would not name the casinos or hotels involved.

The operation follows a spate of murders and the bashing of a veteran operator of casino VIP junkets that had many worried the Chinese territory could see a return to the violence that rocked it the late 1990s. That's when triads, or mafia groups, unleashed a wave of car bombings, assassinations and gangland killings as they battled for control of lucrative casino VIP rooms in the final days of Portuguese colonial rule.

"This was an annual operation held jointly by Hong Kong, Macau and Guangdong," said Jacqueline Chan, a police spokeswoman. "The operation didn't have any specific target. Rather, it was aimed at preventing and tackling crime."

Macau, bordered by the southern Chinese province of Guangdong and an hour by ferry west of Hong Kong, is the only place in China where casino gambling is legal.

After Portugal returned Macau to Chinese rule in 1999, violence abated and the city overtook Las Vegas Strip as the world's most lucrative gambling market, raking in $33.5 billion in casino revenues last year, a 42 per cent increase over the year before. Revenue growth has tailed off in recent months as the Chinese economy slows.

Companies operating casinos in Macau include Las Vegas Sands Corp., Wynn Resorts Ltd. and MGM Resorts International.

The recent outburst of violence includes an attack in June on Ng Man-sun, who is recovering in hospital after reportedly being beaten by six men in his own hotel in a dispute over ownership of the property with his ex-lover. Ng is a longtime operator of VIP casino junkets, which lend money to gamblers.

That was followed in July by the deaths of two mainland Chinese men at the five-star Grand Lapa hotel and the killing of a mainland Chinese woman in a residential neighbourhood near the Venetian Macao casino. The killings remain unsolved.

Hong Kong police said last week they raided 21 locations in the Asian financial centre as part of the operation, arresting 130 people suspected of money laundering, illegal gambling and prostitution. Cash, cars and watches were seized in the raids.