I just happened to be travelling on the London underground when first reports of the terror outrage on Westminster Bridge began to trend on social media.
Within minutes we were informed our train would not be stopping at Westminster station - it soon became clear this was part of a massive, well planned and well co-ordinated security clampdown in the event of such an attack.
Walking towards the scene of the assault, helicopters hovered overhead, while hundreds of policemen, many armed, blocked various access points to the bridge.
It was a pleasant spring day in the capital, and this part of London, as is the norm this time of year, was swarming with tourists. Maybe that added to the sense of unreality and detachment on the nearby streets, given that victims were dead and dying only a few hundred yards away.
Soon word filtered through that the number of dead and injured had made this most innocuous of places a scene of carnage. The shock on the faces of some of the policemen - who had just learned one of their colleagues had been stabbed to death - told its own story.
Onlookers were moved farther away, as the wail of ambulances bringing the dead and injured to hospital drowned out all other noise.
As always with such attacks, the fear was some form of secondary outrage - whether it be a bombing or a shooting - would erupt in the most unlikely of places.
The assault was most dangerous because it was so random. It mirrored similar attacks in Berlin, Nice, and elsewhere. It is the kind of terror almost impossible to defend against. The most potent weapon the attacker had at his disposal was a willingness to kill and be killed. Another powerful force in his armoury was a total disregard for innocent human life.
A determination in such circumstances, to keep on driving a truck or a car, while mowing down men, women, and sometimes children, shows a mindset almost impossible to confront. Therefore a key tension factor must remain. How does one of the great cities of the world allow 'normal' life to function, while at the same time implementing massive, and highly disruptive, security operations?
And indeed it is a moot point if such a defensive strategy can really act as a deterrent in the face of a 'lone wolf' assailant, determined to kill as many as possible, while motivated by a cocktail of religious fervour and hate.
Apart from the viciousness of the event itself, the fact it took place so close to the national parliament hit a particular chord in the British psyche. The very essence of its democratic system seemed to be subject to some kind of peril; it provoked a sense of outrage and anger across the political spectrum.
Of course that is why this particular location was chosen for such a blood-stained attack - an attempt to pierce at the heart of governance and democracy as it is understood in this part of the world.
What has been described as the "great war of civilisations" is mirrored in the vast vacuous gulf which has opened up between the West and much of the Islamic world. It has its roots deep in history, stretching back to events such as the Crusades. Then the armies of Europe invaded the Middle East in a vain attempt to rid the Holy Land of a Muslim presence.
The great divide between the Christian world and Islam would continue in a host of ways through the centuries. In more recent times there were tensions over Middle Eastern oil during the energy crisis of the 1970s, and events such as the 2003 Iraq invasion, and the ongoing chasm caused by the Israel and Palestinian divide, continue to fan the flames of discord and division.
Yet central to a simmering sense of grievance in parts of the Arab society is that too many Muslim countries still remain either an autocracy, or a dictatorship. Excessive power is invariably vested in an elite, while frustration, anger, and resentment, among the masses continues to fester.
In this situation it is all too easy for the disaffected to take comfort in the zeal of religious extremism, coupled with a tendency to blame European liberal democracies, and the US, for the near insoluble political problems in many Islamic countries.
The threat from attacks is one of the most intractable problems of our age. In the meanwhile, Londoners, and the Irish living in the city, need to be on high alert.