Chinese education authorities have gone high-tech to catch cheaters as millions of high-school students take their "gaokao", the annual university entrance exam seen as key to landing a lucrative white-collar job.

So high are the stakes and so competitive is the exam that some students resort to cheating.

Over the years, students have used wireless cheating devices disguised as erasers, belts and watches. Some also use tiny earpieces to communicate with accomplices helping them outside the exam room.

The innovations have forced authorities to step up their game in response.

Exam centres this year have deployed metal detectors, facial and fingerprint recognition technology, cellphone-signal blockers, wireless detectors and even drones in their fight to root out cheating, media reported.

Even before the exam kicked off in earnest on Tuesday, security officers had arrested 52 people nationwide. Cheaters and their accomplices face up to seven years in jail.

Universities in Shandong province have banned students from taking leave on Tuesday and Wednesday, to prevent any of them from posing as real exam takers, the official Xinhua news agency said on Wednesday.

Cheating is more common for papers that include multiple choice questions such as mathematics and English.

Some papers, however, such as essay-writing, confound the cheats.

Essay-writing is an important section of the Chinese paper, and topics vary from province to province.

In Beijing, students this year had to write about either their vision of China in 2049, the centennial of the foundation of the People's Republic of China, or about relationships.

Students in nine provinces were told to introduce China to foreigners using key words like "Belt and Road initiative", "bike-sharing", "high-speed railway" or "mobile payment", in a test of their knowledge of economic trends.

The province of Jiangsu, notorious for tough exams, wanted students to expound on vehicles, while neighbouring Zhejiang province demanded 800-word essays on books.