The head of the UK's armed forces has urged Britons not to let terror fears "stop the British way of life".
Security has "intensified" amid concerns that remembrance events in London were targets of a potential attack.
But General Sir Nicholas Houghton said Britain must not "succumb to any sense that there is a terrorist threat" and should carry on as normal.
The Chief of the Defence staff told BBC One's Andrew Marr Show: "I think the mood is definitely different this year but the biggest thing that makes the mood different, I think, is the intensity and the poignancy about it born of the aggregation of the 100 years commemoration of the First World War, the 70 years of D-Day, I think the end of combat operations in Afghanistan.
"But, certainly the proximity of the sense of threat for this weekend, which has intensified the nature of the security that's attendant on it, has sort of contributed to quite a different feel about this year."
Asked about concerns people may feel about attending services across the country today, he replied: "In an absolutely unreserved way I would say we have just got to keep continuing our normal life.
"The last thing that we at all would want to do is succumb to any sense that there is a terrorist threat there that is at all going to stop the British way of life."
Counter-terror police detained four men, aged 19 to 27, on Thursday night at locations across west London and High Wycombe in the Thames Valley area.
It is understood the men were plotting an attack on British soil. The timing of the arrests raised fears that Remembrance Sunday events in London were alleged targets.
A 19-year-old, understood to be called Yousaf Syed, was arrested at an address in High Wycombe at the same time as a 22-year-old was arrested in Hounslow at 8.45pm on Thursday. Firearms officers assisted with both arrests.
A 25-year-old was arrested in Uxbridge at 2.55am on Friday while a 27-year-old was arrested in a car in the street in Southall at 8.30pm on Thursday.
A Metropolitan Police spokesman said: "Warrants for further detention have been granted for all four men for a further seven days."
Gen Houghton said the threat to Britain from international terror groups was an "enduring one" but was being successfully controlled.
He warned that the military campaign to degrade Islamic State (IS), also known as Isis and Isil, could "take a long time".
The United States confirmed last night that it had carried out air strikes targeting IS leaders in northern Iraq.
Gen Houghton said he could not "absolutely confirm" that IS leader Abu Bakr al-Baghdadi was among those targeted but warned that the terror group would continue to "regenerate" without a political solution on the ground that stamped out its "warped" ideology.
"What I wouldn't want to do is rush to the sense that the potential death of one of their totemic leaders is going to create some strategic reverse in Isis," he added. "They will regenerate leadership."
Gen Houghton insisted that Britain had achieved its mission in Afghanistan.
He told the programme: "The prime strategic purpose of our being in Helmand, and all international security forces in Afghanistan, has been achieved - that of denying Afghanistan as a safe haven for terrorism, such that the streets of the United Kingdom are safer.
"Indeed, in the last 13/14 years there has not been a single international terrorist attack launched from Afghanistan."