Tom James, 32, was led away by police for questioning after Kenyan officers swooped on his home in Nairobi on Tuesday afternoon.
Police allegedly found al-Shabaab literature and books during the raid, which began when a team of officers wielding AK47s knocked on his front door at 10.30am with a warrant.
Up to 20 plain-clothed police officers then searched the house for four hours.
Officers were seen carrying a number of framed maps and large white boards with notes written on them out of the house and placing them inside a car.
Mr James, wearing tan chinos and a blue shirt, was seen walking barefoot around the driveway of the house looking dazed.
Police prevented him from coming down to the front gate to speak to friends waiting outside.
A convoy of cars eventually left the property later in the afternoon with Mr James sat inside a Land Cruiser with blacked out windows.
He was taken to the Kenya Police Criminal Investigation Department for questioning.
An officer involved in the investigation said Mr James had not been arrested but was “helping us with some things”.
Mr James, who speaks some Arabic and Swahili, previously worked in London as an events organiser for The Economist.
Before that he worked as an English teacher in Syria while learning Arabic at Damascus University.
He has also worked as a freelance journalist across Syria, Lebanon and Iraq.
M&C Saatchi conducts activities in Somalia known as psychological operations, or "psyops" which are campaigns to convey particular messages to people to influence their behaviour.
M&C Saatchi, which was launched in 1995 by brothers Maurice and Charles Saatchi, did not respond to requests for comment.
A spokesperson for the Foreign and Commonwealth Office said: “We are urgently seeking information from Kenyan authorities following reports that a British national has been detained in Nairobi.”
Read more: http://www.telegraph.co.uk/news/2016/10/11/kenyan-anti-terror-police-raid-home-of-british-pr-executive-accu/