By Katy Migiro

NAIROBI (Thomson Reuters Foundation) - A Kenyan police officer has been jailed for 20 years for raping a 13-year-old girl, despite saying they are now married, in a case campaigners heralded as a breakthrough in justice in a country where such convictions are rare.

It is one of 11 child rape cases that Kenya's high court ordered be re-opened in 2013 after a rescue center sued the police for failing to investigate hundreds of cases brought to them - instead demanding bribes from and even locking up girls attempting to make reports.

"This is a major breakthrough," Josephine Mongare, chairwoman of the Federation of Women Lawyers in Kenya (FIDA) told the Thomson Reuters Foundation.

"There is hope that nobody is above the law."

Policeman Joseph Mutua was convicted of raping the minor in 2010, despite saying they had since married in a traditional ceremony, he had paid dowry for his bride and they were living together in Mombasa, on Kenya's coast, with their son, according to the judgment released on Wednesday.

Sex with a girl under the age of 18 is an offense in Kenya, although child marriage is common among traditional communities.

"The accused had lured the complainant (the girl) into the sentry box at the gate to the District Commissioner's office and thereafter defiled her," the judgment said in its summary of the prosecution's case.

"She felt a lot of pain and asked him to leave her alone."

The girl's family applied to the Office of the Director for Public Prosecutions (ODPP) for the case to be withdrawn saying the couple had since married, the judgment said.

The girl and her mother testified in support of Mutua but gave contradictory evidence as to which part of the country the couple were living in.

"It is clear that (the) accused, the complainant and the complainant's mother aren't reading from the same script," magistrate Oscar Wanyaga said in his judgment.

"They can't agree on simple things like where they reside as a married couple... This is just a story created to win (the) accused his freedom."

In Kenya, it is common for victims' families to settle cases out of court, accepting money or goats as compensation - leaving the perpetrator free to reoffend.

"It all has to do with poverty and valuing money more than the life of their own daughter," said Mercy Chidi, who runs Ripples International rescue center in Meru, 240 km (150 miles) northeast of the capital Nairobi, and helped the girl to file the case six years ago.

One in three Kenyan girls experience sexual violence before the age of 18, a 2012 government survey found, but they rarely report it due to stigma and lack of faith in the police.

"This is one of the cases that we had that was really tough, a police officer investigating their own," Chidi said, referring to the 11 cases that she succeeded in getting re-opened.

This is the third that has since resulted in a conviction.

Conviction rates have improved since 2011 when the ODPP became an independent office with the power to look at files before trials begin and ask for additional evidence, said Mongare of FIDA.

While Kenya's estimated 80,000 police officers often lack resources, like transport, to investigate cases, it is shifts in attitudes that are key, experts say.

"We cannot condone impunity or indiscipline amongst ourselves," said police spokesman George Kinoti.

"When this officer was convicted, it's an example to the rest that by virtue of being a police officer, you are not immune to judicial process."

(Reporting by Katy Migiro @katymigiro; Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women’s rights, trafficking, property rights and climate change. Visit http://news.trust.org to see more stories.)