A group of Indian Catholic nuns are staging a rare street protest in the southern state of Kerala demanding justice after an alleged sexual assault of a nun by a bishop.

Police have called the bishop named in the complaint for a second round of questioning next week. The bishop has denied wrongdoing.

he case comes at a time when the Christian community, which accounts for 19 per cent of the state's population, is reeling under an erosion of trust as sexual abuse cases involving the clergy pile up.

Five priests were arrested last month in two different sexual assault cases, while last year, a Catholic priest who was trying to flee the country was arrested after a minor he is accused of having sex with gave birth.

The nun in a letter said she wanted the Vatican to intervene. She said she was forced to make matters public after several attempts to seek justice from within the church failed.

The protest, now in its sixth day in Kochi, Kerala's financial hub, is gathering momentum as the locals, activists, writers and politicians come out in support.

A man, who is on an indefinite hunger strike, lay in front of the stage where the protesting nuns sat. A nun coordinating the protest dismissed allegations that they were trying to disgrace the church.

'Our fight is for the truth. We will not back out unless the truth is established and the nun is given justice,' she told Reuters.

The Catholic Church in Germany acknowledged a 'depressing and shameful' legacy of sexual abuse on Wednesday after a leaked study said clerics had abused thousands of children over 70 years.

The leaked study was published on the day that Pope Francis, who has made several attempts to tackle a spreading sexual abuse crisis that has badly tarnished the Church's image worldwide, summoned senior bishops from around the world to the Vatican to discuss the protection of minors.