BEIJING, June 4 (Xinhua) -- As millions of Chinese students gear up for the national college entrance examination, education and police authorities Thursday issued stern warnings against cheating.

    The Ministry of Public Security has instructed police departments across China to make detailed plans to ensure the security of the exam, which falls on June 7 to 9 each year.

    The police would be on high alert for any cheating, such as the use of electronic devices by both students and people outside the exam site to exchange questions and answers, said a statement from the ministry.

    The Ministry of Education Thursday announced four investigations that involved the production and sale of devices to be used during the exams, using them as a warning.

    The police in northeastern Jilin Province uncovered four underground workshops late last month, where more than 100 sets of devices were found. At least seven people involved were detained.

    On May 31, police in central Hunan Province detained a person surnamed Sun, who allegedly published online information about "selling" exam papers and answers to be used for the province's examination.

    But further investigation found Sun was a fraud because he did not possess any official information of the exam, an education ministry spokesman said.

    In a similar case in eastern province of Fujian, a man surnamed Chen was detained by police.

    The ministry said cheating students would be barred from sitting the national examination for two years. It also published telephone numbers for the public to report complaints.

    Police departments were asked to assist schools to ensure exam security, for example, to prevent the theft of exam papers while they are printed and delivered.

    About 10.2 million people registered to sit this year's exam, down 3.8 percent year on year, possibly because college graduates are having a hard time finding jobs as posts are axed in the economic slowdown.

    However, seats at colleges and universities are still hotly contested because a college or university diploma often means better job opportunities and income in the long term. This has been blamed for motivating some people to cheat.