The 28-year-old man was arrested on the morning of Tuesday, 2 November at an address in east London on suspicion of disseminating terrorist publications (contrary to Section 2 of the Terrorism Act 2006).
He was detained under the Police and Criminal Evidence Act (PACE), and taken to a London police station.
The man was at one of two addresses in east London which were visited and searched by SO15 officers.
The ongoing investigation relates to alleged extreme Islamist material posted on social media.
+ Communities defeat terrorism, and information from the public is vital to counter terrorism investigations. If you see or hear something unusual or suspicious and think someone may be engaging in terrorist activity, trust your instincts and act by reporting it in confidence at gov.uk/ACT. Always dial 999 or in an emergency.