Nine students were detained for allegedly receiving the signals and three university students, Yongkang natives, had been held for providing the answers, the report said.

The scam was exposed on the morning of June 7, the first day of the exam, when a surveillance patrol car picked up radio transmissions near the Yongkang No1 Middle School. Police located the source of the transmission as a cafe across the river and later detained 15 suspects there.

Five other suspects were caught in the afternoon when another source of transmission was found.

A third transmission source was discovered one day later and another suspect was detained.

The 21 suspects identified nine students who were sitting the exam. Twenty-one mobile phones, six walkie-talkies and six computers were confiscated.

One of the implicated students, surnamed Xu, admitted to sharing the 10,000-yuan cost of the complete test answers with two other students and paying another 1,600 yuan (HK$1,819) for the transmission devices, including a transmitter, a receiver, walkie-talkies and a tiny earphone, according to Xinhua.

Police said the answers the students paid for were the standard ones to the national university entrance exams, whereas Zhejiang had used another set of tests for the exam.

Under the rules governing university entrance exams, students who cheat will be banned from taking them for two years. Cheating also leaves a permanent mark on a student's record, which could affect his or her future credibility.

The Ministry of Education said 2,645 students were caught cheating during the exam this year, down by about 800 from last year.

But in the fierce competition of the mainland's job market, many feel it is almost impossible to find well-paid employment without a university degree. To ensure their children get into universities, well-connected parents will do everything in their power.

It is common for parents to buy hukou, or residence permits, in an area where the threshold of entering a quality university is lower. Some go even further by arranging for people to take exams on behalf of their children.

In Gansu's Tianshui , three police officers, a civilian worker in a police station and a high-school teacher were arrested on Friday for their involvement in a well-calculated entrance-exam scam that had already seen four officials in Shandong and Gansu arrested late last month.

Police officers Liu Xiaobin, Hou Zhaopeng and Jia Xudong and civilian police worker Zou Rong , who worked in police branches in Maiji district of Tianshui, allegedly took bribes to fabricate household registration and ID cards for 31 illegal examination takers from Shandong, Xinhua quoted Yang Ruifeng, Tianshui government spokesman, as saying.

Zhang Hongju , a high-school teacher, was also arrested for allegedly helping the people apply for the examination.