Scotland Yard is tracking a number of people it fears could be planning terrorist attacks on London, Assistant Commissioner Andy Hayman has said.
Londoners should be vigilant, the Met's assistant commissioner says
He told the Guardian that no plot had been discovered but that the city was an "iconic site" for another attack.
"I don't want to scaremonger but it has to be said, when you look around the world and at the prominence of London, that the threat is real," he said.
He also admitted difficulty for police seeking the Muslim community's trust.
"There has been progress but starting from a regrettably low baseline," Mr Hayman said.
"We have close links with the Muslim community but the next step is getting them to share information.
"I fully understand how difficult that is - the repercussions of arrests and so on - but that has to be weighed against the mass loss of life that could result from further atrocities."
Mr Hayman told the Guardian that the Met remained "active in covert operations. We have a number of people who are of interest".
None of those is understood to be directly linked to the 7 July bombings or the failed attacks two weeks later.
Mr Hayman said: "We have to be vigilant, but you can't predict where or how or when [terrorists] will try."
FBI Director Robert Mueller said on Wednesday that the US feared attacks by both home-grown and foreign terrorists.
He told BBC Radio 4's World Tonight: "Individuals who can undertake attacks who have grown up in the United States and can undertake attacks in the United States, we're concerned about that, but we are also concerned about those who will come in from overseas as they did on September eleventh."