BEIJING (Reuters) - Chinese police in the restive mountainous region of Tibet beat up to 50 monks who had tried to complain about the beating of one of their colleagues, a rights group said.

Four of the monks had to be hospitalised, the Free Tibet Campaign said in an emailed statement of an incident which happened earlier this week at the Kirti monastery, citing an unnamed source.

"The monk had left the monastery earlier that day, having obtained the relevant permission from the monastery authorities. As the monk returned to the monastery a short time before dark, he was stopped by Chinese armed police," the group said.

"According to the source, the returning monk was beaten so badly by the armed police that he was bleeding when he managed to return to the monastery."

When other monks went to the police station to demand an explanation, they too were beaten up, the statement added.

It was not possible to independently verify the report but China regularly denies claims of repression in Tibet.

Beijing laid out its case in a "white paper" issued on Thursday, amassing statistics about literacy, education and religion to argue that Tibet had enjoyed a cultural revival since the Chinese Communist Party took control in 1950.

Tibet was rocked by renewed protests in March, blamed by China on exiled Tibetan spiritual leader the Dalai Lama. He denies the charges.

China's crackdown on the March unrest drew widespread international criticism.