The suspect, known only a CF, is only 24 but allegedly has tried to recruit Britons to join the radical Islamist cause and had previously tried to travel to Afghanistan for terrorist training.
CF is also suspected of being a veteran of the Somali extremist group al Shabaab. A Home Office lawyer told the newspaper CF allegedly wanted to "re-engage in terrorism-related activities either in the U.K. or Somalia."
The Sunday Telegraph said CF was picked up after allegedly passing through Olympic Park five times despite a ban on such traffic in the run-up to the games. Officials called his detention the most serious security alert yet at the venue.
His arrest was disclosed shortly after 14 suspects were arrested on charges related to alleged terror plots in Britain.