Thomas Sutcliffe: Two minutes of punctuated silence as we remember

As silences go, it's never perfect; not so much the absence of sound as the presence of a great collective restraint. For two minutes – from the first stroke of Big Ben at 11am to the sharp-edged crack of the gun that marks its end – the crowd keeps its peace while it thinks of war, interrupted only by the occasional crackle of police radios and the click of camera shutters.

As usual, the silence took much of its texture from the noises that preceded and followed it, the lovely grainy pulse of precision marching, squawks of command from sergeant-majors – trying to out-do their colleagues in vocal zeal – and the military bands, filling the space between the buildings with the rounded blat of kettle-drum and brass.

Now and then, a wet patter of clapping would trickle down Whitehall as a new contingent of veterans marched into place and were acknowledged. What was different this year was that British troops were again at risk – in a war where the front line may turn up anywhere without warning.

After the laying of wreaths by the Queen and the Prime Minister and a brief prayer from the Bishop of London, the crowd joined in the singing of "Oh God, our help in ages past", a communal insistence that divine security would cover all eventualities. "Sufficient is thine arm alone," they sang, "And our defence is sure".

Worldly powers were taking no chances, though, adding a defensive line of their own to that of the Lord. If terrorism is the true adversary in this conflict then it was obvious that this ceremony – one of the only occasions in the year when the entire British establishment gathers in public at a predetermined time – would offer the enemy what the Pentagon calls a "target-rich environment".

On the roof of the Foreign and Commonwealth building the classical nymphs were flanked by men with baseball caps and binoculars, scanning the crowd for signs of trouble. If public attendance had slackened, the shortfall had been made up by lines of extra police on Whitehall. No planes flew overhead during the ceremony.

Earlier, the morning, newspapers had reported that the UK was moving to a state of public emergency, but here, such thoughts seemed oddly distant. This was a moment when the current crisis could be forgotten, temporarily obliterated by an act of remembering dedicated to struggles that are now over.

Remembrance Sunday has always been an occasion for rituals, not speeches, and those rituals are all about the past, about combats concluded and believed, with hopeless optimism, to be conclusive. Elsewhere, Tony Blair might have posthumously recruited "The Glorious Dead" to a new cause, and added the victims of 11 September to the roll-call of commemoration. But here in Whitehall were few reminders that new names might soon be added to the lists, and even those would have been easily missed by a crowd that had come for different purposes.

From a balcony overlooking the Cenotaph the American Ambassador, William Farrish, attended the ceremony for the first time, and Joe Callan, the New York fire chief, marched beside British firemen at the tail end of the parade – picked out by his white cap and the fact that he alone, out of the many thousands of marchers, saluted the memorial. As in most years, it took nearly three-quarters of an hour for the war veterans to march past – a procession that should be dwindling year by year, as quieter deaths diminish it. As soon as the ceremony was over the knowledge returned that next year much younger survivors may swell its numbers.