GUANGZHOU, Jan. 7 (Xinhua) -- Police in south China's Guangdong Province have cracked a slew of online fraud cases involving malicious mobile apps, busting 21 rings across the country and detaining more than 600 suspects, authorities said Sunday.

They also froze illegal revenue of over 100 million yuan (about 15.3 million U.S. dollars) and seized more than 400 computer servers in the coordinated operation carried out in 13 provinces, autonomous regions and municipalities last month.

The operation came after Guangdong police noticed in August last year the emergence of suspicious mobile apps that often had sexually-suggestive names.

After investigation, they found suspects used the apps to lure smart phone users to pay for "exclusive" dating services or porn videos, raking in as much as millions of dollars every month.

The total amount of money involved in the frauds exceeded 1 billion yuan (about 153 million U.S. dollars), according to the police.

"This is the largest raid against malicious mobile apps that Guangdong police have launched in recent years," said an official with the cyber division of Guangdong police, vowing to intensify a crackdown on online crimes to ensure a secure cyber space.