The Macau police say they have broken up two illegal gambling syndicates that have taken hundreds of millions of dollars in bets on the World Cup.
Officers arrested 26 people during two raids on a hotel in the Chinese gambling city.
The illegal bookmakers had been taking telephone and online bets from across the world, police said.
Evidence uncovered in the raids suggested that one gambler had placed a single bet of more than $5m.
Police officers raided the hotel on Thursday and again on Friday.
In the first sweep, they discovered a gambling syndicate that had been operating out of three rooms in the hotel. Officers found betting slips, details of gambling accounts and a large amount of cash.
Those arrested come from mainland China, Hong Kong and Malaysia. Handcuffed and with black bags over their heads, some of the suspects were paraded before the media.
Gambling is mostly illegal in mainland China, although it is allowed in Macau and, to a lesser extent, Hong Kong.
However, the two former colonies have been cracking down on illegal gambling involving the World Cup football tournament, now taking place in Brazil.
Hong Kong police have set up a task force with Macau and the authorities in Guangdong, the next-door Chinese province, and are working with Interpol across eight Asian countries.
The authorities believe criminal gangs have become involved in sports betting in Asia because of the large profits that can be made.