A campaign urging London's communities to help fight terrorism is being launched by the Met.
Posters will appear on the Tube
It wants people to be aware of suspect packages and unusual activity in places like flats or garages.
The month-long campaign, with the slogan "If you suspect it, report it", will be featured on the radio, on Tube posters and in newspaper adverts.
Peter Clarke, deputy assistant commissioner in the Met, said: "Communities can defeat terrorism."
A Met Police spokesman said of Monday's launch: "The focus [of the campaign] is to encourage the public to be vigilant and to contact police with any suspicions.
"The first strand urges the general public to be aware of their surroundings, with posters urging people to look out for unattended or suspicious bags or packages.
"The advice to passengers on public transport is to ask whom the unattended or suspicious bags or packages belong to and to inform station staff or dial 999.
"The second strand encourages the public to contact the confidential anti-terrorist hotline on 0800 789 321 if they notice any unusual activity at lock-ups, garages or flats or have suspicions about particular vehicles."
Mr Clarke, who heads the Met's anti-terrorist branch, said: "The Metropolitan Police Service is working harder than ever to keep London safe from the threat of terrorism but we very much need the public's help to reduce the danger.
"I would ask people across London to think very carefully about anyone they know whose behaviour has changed suddenly. What has changed - could it be significant?
"What about the people they associate with? Tell us what you see and what you know, and let us decide if the information you have is valuable or not.
"I would also reassure anyone who may be reluctant to contact the hotline, that it does not matter if what they tell us, turns out to be innocent."