Chinese police have broken up two major child trafficking gangs and freed 181 children, officials say. Authorities arrested 802 suspects on Monday in an operation across the country, the Public Security Ministry said in a statement. Kidnapped children are often sold for adoption, or as labour and household servants. Child-trafficking has become a serious problem in China and critics blame the one-child policy and lax adoption laws. These policies, some say, have created a thriving underground market for buying children. A traditional preference for male heirs in China has created a thriving market for baby boys, says the BBC's Martin Patience in Beijing. Women and girls are often abducted to be labourers or wives. In the latest operation, the children were rescued from traffickers in 15 regions and provinces, including Hebei, Shandong, Henan, Sichuan, Yunnan. Investigations that led to the current round of arrests began in December 2011 when four suspects were caught in Henan province while attempting to sell four babies. A ''most-wanted'' suspect, Shao Zhongyuan, was caught in Pingyi county, Shandong province, the ministry said. He was alleged to be from a gang that trafficked more than 100 children. Several other key suspects were also detained, police said. The ministry released a report in March 2011 saying that police rescued tens of thousands of abducted children and women. It highlighted one raid against a gang trafficking Chinese women to Angola for prostitution, adding that 19 women were rescued and 16 people were arrested. Greater freedom of movement as a result of China's economic reforms is thought to have also made it easier for trafficking gangs to operate.