Between 11pm last night and 5am on Tuesday, 640 police officers, transport workers, emergency services, and a hundred volunteers were on hand in the business district to carry out a series of mock attacks.
Monday night's exercise in Paris - the latest in a series of simulated terror assaults - saw a mock attack on the usually-crowded business district of La Defense.
It was designed to replicate an attack at peak time in the country's financial centre.
"Even though everyone knew it was just a simulated attack, the sound of gunfire and the surprise of it was enough to make you freeze," a journalist from Le Parisien newspaper wrote.
One of the volunteer hostages told the paper that the exercise was "very realistic".
"They caught us and gathered us in a room, making us sit and stare at the ground. They were saying Islamic chants, called us bitches in miniskirts, and telling us we were going to die as martyrs with them," she said.
The simulated attack marks the latest of its kind in a country on edge after a series of high profile terror attacks, most notably in Paris and Nice, where over 200 were killed in two devastating attacks.
The western city of Rennes held a similar set of large scale terror exercises on Friday, several cities in eastern France did the same thing not long before, and Lyon's mock attacks before the Euro football tournament made news during the summer.
These mock attacks are said to help police prepare for anything from hostage situations, mass evacuations, bombings, and chemical attacks.
Terror expert Roland Jacquard told the Direct Matin newspaper that French people "are still not sufficiently prepared" for potential terror attacks.
"It will take years, but faced with the constant risk attitudes will change," he said.
Other measures to prepare for terror include a new government-backed app called SAIP that alerts users to any terror attacks in real-time, with the goal of sending out alerts within fifteen minutes.
Schools in France, which have been singled out as terror targets, are also set to stage mock terror attacks, which was recently made a compulsory exercise for all pupils over the age of three.
Students are also set to be getting extra education in things like first aid, while teachers will get trained in how to deal with crisis situations.