Kenya's independent police oversight body has launched an investigation into the death of a man who had been detained "for false publication", police have said.

Albert Ojwang was arrested for a post on X in the western town of Homa Bay and then driven 350km (220 miles) to the capital, Nairobi, his father Meshack Opiyo told journalists.

"While in custody, the suspect sustained head injuries after hitting his head against a cell wall," a police statement said. He was rushed to the hospital, "where he was pronounced dead on arrival".

The director of rights group Amnesty International's Kenya branch told the BBC that the death of Mr Ojwang, described as a teacher and blogger, was "very suspicious".

Amnesty said in a statement that his death "raises serious questions that must be urgently, thoroughly, and independently investigated".

Senior police officer Stephen Okal is quoted by the Star newspaper as saying what happened in the cell was "an attempted suicide".

It is not clear what the charge of "false publication" referred to, but Mr Opiyo told online news site Citizen Digital that the arresting police officer said "Albert had insulted a senior person on X", the social media platform.

Referring to the circumstances of his arrest, Amnesty International Kenya director Irungu Houghton said it was "quite shocking" that Mr Ojwang was not booked in at the local police station after being detained, but was instead taken on a long journey.

He called on the independent investigators to secure what he described as "the crime scene" at the police station in Nairobi.

The police said that Mr Ojwang was "lawfully arrested".

His detention and death come at a time of rising concern about how some government critics are being treated.

Last week, software developer Rose Njeri - who created a tool to help people oppose a government finance bill - was charged with violating a cybercrime law.