Security has been tightened at London's major train and tube stations after officers were called to a bomb scare last week.
The Metropolitan Police told getwestlondon that commuters can "expect to see more officers, including armed police in and around the transport hubs".
The Joint Terrorism Analysis Centre has raised the threat level for London transport to "severe", meaning "an attack is likely" and police have urged commuters to be vigilant and report anything they feel is suspicious.
Patrols are already in place by British Transport Police on the Underground network, but have been re-enforced with armed units after a "suspicious item was found" on the Jubilee line at North Greenwich tube station on Thursday (October 20).
The discovery led to a 19-year-old's arrest in Holloway Road on Friday (October 21) in a dramatic swoop involving the Army's bomb squad and and counter terrorist detectives.
A Scotland Yard statement read: "The 19-year-old man was arrested on suspicion of the commission, preparation and instigation of terrorism acts, under Section 41 of the Terrorism Act 2000.
"He has been taken to a London police station where he remains in custody."
Following the arrest, Mayor of London, Sadiq Khan , said: "Thanks to the outstanding professionalism of the Metropolitan Police, Transport for London staff and the British Transport Police, this situation was dealt with swiftly and safely and no injuries resulted.
"I have been in constant contact with the Metropolitan Police Commissioner and the TfL Commissioner throughout the past two days.
"Keeping Londoners safe is my highest priority.
"I am urging all Londoners to check with TfL before they travel, and to remain calm and vigilant at all times."
"Over the next few days you will see more officers in and around transport hubs to provide reassurance about public safety."