People are being urged to call a hotline as the Metropolitan Police launches an anti-terror drive.
Poster ads will appear all over London this week
The move comes as Foreign Secretary Jack Straw prepares to join European colleagues on Monday to discuss ideas to combat international terrorism.
EU police chiefs are meeting in Dublin on Monday to talk about terrorism.
Planning for the Olympic Games in Athens and the European Football
Championships in Portugal is on the agenda.
The police chiefs will meet colleagues from states joining the EU this May and are due to hear from the heads of Interpol, Europol and Cepol, the European Police College.
The UK's anti-terror hotline is part of a Life Savers advertising campaign being launched by the Met on Monday.
Police want people to report suspicious behaviour on 0800 789321.
Assistant Commissioner of specialist operations, David Veness, said: "We want to make London one of the most hostile environments to terrorists."
He said: "We have massively increased the effort and resources we are devoting to security operations and counter-terrorist investigations.
"But we still very much need help to reduce the danger posed by terrorists."
He added: "All information... is researched and investigated. Let us decide if the information you have is valuable or not."
Meanwhile EU ministers meeting in Brussels will study ideas on enhancing counter-terrorism efforts which will be presented to European leaders on Thursday and Friday.
A package drafted last week would create an anti-terrorism co-ordinator.
A draft declaration being considered labels terrorism a "key threat" to the EU and commits the 25 current and soon-to-be EU members to "act jointly in a spirit of solidarity if one... is the victim of a terrorist attack."
It includes a NATO-like pledge to mobilise "all the instruments at their disposal, including military resources," to prevent attacks or assist
But differences remain on intelligence sharing between national police and security organisations.
Also on Monday, intelligence officials from France, Britain, Germany, Italy and Spain will meet in Madrid to review the March 11 attacks in the city and discuss how to improve cross-border co-operation.
The developments come after Metropolitan Police Commissioner Sir John Stevens said the Madrid train bombings which killed more than 200 people should act as a wake-up call to the rest of the continent.
He told the BBC intelligence on terrorism could be shared along similar lines to those used by Europol, which tackles organised crime.