"I've never seen anything like this... It was a night of nightmare," Mr Philippe Jouvain, a hospital director, tweeted as his emergency ward doctors struggled with the blood-soaked aftermath of the Paris terrorist attacks.
The chaos and carnage at one of the French capital's leading hospitals came at the end of the bloodiest night in Paris since World War II.
But the nightmare for security and intelligence services throughout the world is only just beginning. For the events in Paris are a grim reminder of how difficult it is to protect any urban centre from terrorist attacks.
Ironically, Paris is already one of Europe's most heavily policed capitals. Many policemen are deployed along the city's central avenues, the so-called "grand boulevards". Vans full of heavily armed officers belonging to the CRS crack security units are parked discreetly behind most public buildings.
France also has a long tradition of centralised authority, which means that its resources can be quickly deployed. Few other European countries would have been able to impose a state of emergency, order the closure of all places of entertainment and seal the country's frontiers at the same time, yet French President Francois Hollande did all these things, and his orders were carried out almost instantly.
The French mistrust authority, but also worship it. And they respond with defiant collective national pride whenever their country is under threat. The crowds evacuated from the Stade de France stadium in the wake of the terrorist attacks burst into a spontaneous rendition of their national anthem.
Yet none of these national advantages can insulate France from determined terrorists, who retain the initiative because they can choose the battlefield.
The men of violence are no longer interested in targeting government buildings, famous tourist landmarks or main shopping streets like the Champs-Elysees, all of which are already under considerable protection. Instead, they now go after any place where large numbers of people gather, with the aim of killing as many civilians as possible.
The fact that most of the attackers appear to have been suicide terrorists also simplifies their murderous task: Since they don't have to worry about escape routes, terrorists can position themselves in the midst of crowds, boosting the number of casualties even further.
And, by launching simultaneous attacks in a number of places, the terrorists not only retain the element of surprise, but also slow down the response of the security services, another factor which maximises casualties. The killers at the Bataclan, a popular concert venue in Paris, had time to reload their Kalashnikov assault rifles no fewer than three times before they were confronted by law officers.
What's more, terrorists in France now appear to favour a new, deadlier innovation: The capture of large numbers of people who are then held hostage not, as previous generations of terrorists used to do, in order to exchange them as part of a deal, but simply in order to prevent people from escaping, and to butcher as many as possible.
An early version of that occurred in January, when two gunmen stormed into the office of Paris-based Charlie Hebdo satirical magazine and shot dead 12 people while an apparent accomplice killed a police officer and four hostages held at a market shortly after.
This tactic forces the security services to storm such hostage hideouts almost immediately, without preparation; that is what happened with the Bataclan concert hall on Friday night. Sadly, such hurried rescue operations almost never end well: They invariably result in a higher number of casualties, both among hostages and rescuers.
Ways to minimise future attacks do exist. Israel, for instance, now imposes airport-style security checks at all places where large numbers of people mingle, including shopping malls, cinemas, concert halls, markets and even major streets. These are hived off into various parameters, each one with its own checkpoints. The effect has been almost no civilian casualties in years.
However, it is unlikely that the residents of European cities would accept such restrictions.
And even if governments are prepared to pay the considerable costs which such intrusive security measures entail, it is doubtful that they can be applied to great effect: The Israeli measures work on "racial profiling", on separating those who appear to be Jews and are given only cursory checks, from others who look racially different and are subjected to intensive checks.
Yet, no European city - and especially not Paris - can resort to such invidious procedures without unleashing ethnic tensions.
The only other alternative remains that of using the intelligence services to dismantle terrorist plots while they are being hatched. The French have foiled at least three attacks this year alone.
But no government can offer complete protection against any threat, and particularly not against terrorism which resurfaces under different guises.
All that governments can do is what Mr Hollande has already pledged: Ensure that peaceful societies make no compromises with the men of violence, and that the murderers of Paris will be pursued mercilessly.
