Macau customs officials arrested a suspect in connection with the operation of a website which charged users for illegal song and movie downloads.
It is believed the suspect thought he could avoid capture because its server was based in Hong Kong.
The website, fdzone.org, operated on a membership scheme, and generated an income of millions of patacas, Macau customs officers said.
Two other men and two women were also detained for questioning. The customs officers said they believed the four helped launder the website's proceeds.
Those arrested face up to eight years in jail upon conviction.
The arrests were part of a joint action with their Hong Kong counterparts, the Macau officers said.
They said the key suspect was a Macau resident, known on the website as Andygod, who graduated from what they would only say was a 'renowned university outside Macau'.
They said he attempted to use his professional computer knowledge to evade prosecution.
'Its server was in Hong Kong, although the person hid in Macau. The key criminal thought it wouldn't have been found out with this modus operandi,' said Cheng Kin-chong, head of Macau customs' intellectual property department.
'The website had a tight administration; outsiders were forbidden from entry unless they paid and got verified.'
Police raided a home in northern Macau on Thursday and seized five computers, watches and some cash in Hong Kong dollars and euros, which were believed to be illegal proceeds from the website, the Macau customs officers said.
Nearly 900,000 members each paid a fee of between HK$300 and HK$700 in return for access to illegally uploaded songs and movies. The operators of the site had also received HK$5,000 a day from advertising since its operations began in 2003, the officers said.
Hong Kong customs officers confirmed the arrests, but provided no further details.