More than 9 million students sat China's notoriously tough college entrance exams on Thursday, with "high-flyer" rooms, nannies and even intravenous drips among the tools being employed for success.

With just 6.85 million university spots on offer this year, competition for the top institutions is intense, and attempts to cheat are rife -- 1,500 people have been arrested on suspicion of selling transmitters and hard-to-detect ear pieces.

Parents and students this year are also resorting to some outlandish but legal methods to ensure nothing goes wrong in the make-or-break two-day exam.

Students have reportedly been given pre-exam injections and intravenous drips designed to boost energy levels, while girls have resorted to hormone injections and birth control pills to delay menstruation.

"There are situations where girls take pills to delay their periods until after the exams," a gynaecologist at Beijing's Chaoyang Hospital, who declined to give his name, told AFP.

Some of the more affluent parents have rented houses close to the 7,300 exam venues across the country, while so-called "high-flyer rooms" are being offered in the northern port city of Tianjin, according to the state-run China Daily newspaper.

The special hotel rooms -- which cost up to 800 yuan ($126) more than an ordinary room -- are billed as having previously been rented out to someone who scored high points in the exams.

Rooms with lucky numbers such as six -- which symbolises success in Chinese culture, or eight -- which represents wealth -- are also favourites.

"Every year the house rental market heats up ahead of gaokao (the Chinese name for the exam)," Jin Guangze, a teacher from the Beijing Experimental High School told the China Daily.

The exam has also given rise to a new and lucrative industry -- the gaokao baomu -- or "exam nannies" -- who are tasked to look after students during the exam period.

"The nannies are well-qualified with at least a college-level degree," said Jennifer Liu, marketing manager at Coleclub -- an agency that provides household help and has offered the service since 2009.

"They are there to help the students -- cook meals, wash clothes, tutor the students and offer support for their mental well-being," she told AFP.

Liu declined to disclose how much it cost to hire a nanny, but media reports say the service costs an average 4,000 yuan over a 10-day period.

Meanwhile, China's state television CCTV has repeatedly broadcast advice to help students prepare for the exams and has warned against cheating, airing a confession of a remorseful suspect caught for aiding students to cheat.

The nation's public security ministry said in a statement Monday that police had busted over 100 gangs suspected of selling cheating equipment, rounding up 1,500 people with the seizure of some 60,000 devices such as ear pieces.

Exam authorities said they would use wireless signal jammers and frequency detectors to prevent cheating, as well as fingerprint scanners to verify exam-takers' identity.