More than nine million students packed exam halls across China for the opening day of the country's university entrance exam on Friday —with attempts to stop cheating even leading to bans on metal bra clasps.

Students in the northeastern province of Jilin were banned from wearing clothing with metal parts and education authorities installed metal detectors in exam centres to clamp down on “wireless cheating devices”, the state-run Global Times reported.

Authorities have become increasingly concerned about the risk of examinees using devices such as smartphones — some of which have become smaller and easier to hide — as an illicit aid during tests.

Around 9.12 million high school students were registered across China to sit the crucially important two-day exam, known as the gaokao, reports have quoted the education ministry as saying.

Officials aimed to crack down on “sales of high-tech gear for cheating, and gaokao-related fraud”, the Global Times said.