The Metropolitan Police has launched a counter-terror campaign urging people to report suspicious behaviour by calling a dedicated hotline.
The four-week campaign, named "It's probably nothing, but...", consists of adverts in local and ethnic minority press.
There is also to be a radio advert and leaflets are being delivered to 1.4m homes in London.
Deputy Assistant Commissioner Stuart Osborne said: "The terrorist threat to London, and the UK generally, remains real and serious so it is vital that everyone continues to be vigilant and aware.
"We at the Metropolitan Police Service have a key role in countering this threat, whether it is from al Qaida inspired groups or individuals, or Irish dissident republican groups - but we can only do this with the support of all our communities.
"Terrorists live among us. We want you to tell us about anyone or anything you see which is out of place in your normal day to day lives.
"We know you may have concerns about speaking to the police, possibly because your friends or family may find out. But you may well have information which could save lives."
He added that the Met would rather take lots of calls about incidents with "innocent explanations", than receive no information at all.
"Not making that call could mean we miss out on a vital piece of information," he said.
The confidential Anti-Terrorist Hotline can be reached by calling 0800 789 321.