Chinese police said they suppressed campus protests on Tuesday (June 8), after thousands of angry students held a college principal hostage over fears their degrees were set to be downgraded.

Large-scale protests are extremely rare in China, as the Communist Party controls mass movements that may undermine social stability and quickly extinguishes information that seeps onto the Internet.

Undergraduates at Nanjing Normal University's Zhongbei College, in eastern Jiangsu province, were enraged by a plan to merge the school with a vocational college, fearing it would devalue their qualifications as they prepare for China's intensely competitive job market.

They detained the principal on campus for more than 30 hours from Sunday onwards, Danyang city police said in a statement.

Officers were surrounded by some students "who shouted verbal abuse and blocked law enforcement", the statement added.

Social media users posted photos of police using batons and pepper spray on students, and one female undergrad bleeding from the head.

"To uphold campus order ... public security organs took necessary measures in accordance with the law to remove the trapped person, and (the injured) were immediately sent to hospital for treatment," the statement said.

The Jiangsu education authorities suspended the merger plans, which affected five local universities, late Monday evening. But students at Zhongbei College refused to stop, police added.

The hashtag "Nanjing Normal University Zhongbei College students injured by violent law enforcement" was blocked on Weibo by Tuesday afternoon, along with all related images.

But one video circulating on Twitter showed dozens of police and security personnel shoving and yelling at students in a crowded school hall.

Another showed thousands of chanting undergraduates surrounded by lines of officers on a sports ground, as police dragged some out of the crowd.

One student witness confirmed the authenticity of the images, estimating that around 3,000 students and 400 police officers were involved.

"We were not arrested. The school hired auxiliary police who injured, beat, pepper-sprayed, threatened and verbally abused students," a student said, withholding their names for safety reasons.

The Jiangsu Education Department originally announced the plan in March, saying that it was complying with a Ministry of Education directive to transform private colleges into vocational schools.

Such institutions are widely perceived as less prestigious than universities in China.

"It thoroughly deceives and conceals the truth from students. That's why students spontaneously organised (the protests)," the student witness added.

"Zhongbei College fees are really expensive, around 17,000 yuan (S$3,500) a year... who would spend that much on a vocational college?"

The five schools issued separate statements in recent days, assuring current students that they would still receive university diplomas despite the change.

China is expected to have a record nine million university graduates this year, who will face stiff competition for jobs in a sluggish economy.