Jerome Starkey, 35, was locked in a cell overnight after arriving in Kenya from the UK late yesterday.
"Just been arrested at the airport on orders of security services," Starkey tweeted early on Friday morning, posting a photograph of an untidy room with metal bunk beds inside the airport terminal.
Officers told Starkey a "security block" had been placed on his passport but were unable to provide any further information.
An award-winning journalist, Starkey has lived in Kenya since 2012 when he was appointed Africa correspondent for The Times after years working in Afghanistan.
He has been questioned by immigration, detectives and anti-terrorism police but after more than 15 hours in custody had still not been charged by Friday afternoon.
A spokesman for the British High Commission in Nairobi said: "We are in contact with local authorities and are providing assistance to a British man following his detention in Nairobi.