As the first reports of the attack landed on smartphones, droves of EU attendees were on their way to dinner and parties but were stopped at the parliament exit doors by no-nonsense security guards.
"The lockdown will hold until there's a resolution to what is going on outside," a security official said, waving everyone back.
Inside, with the session still ongoing, things remained business as usual, at least initially.
In a show of solidarity European Parliament President Antonio Tajani insisted on keeping the plenary session open, and debate continued as planned until midnight (2300 GMT).
"This parliament will not be intimidated by terrorist or criminal attacks... We will continue to work, strengthened by freedom and democracy against terrorist violence," Tajani told MEPs.
Tajani at 0100 GMT announced an evacuation plan in cooperation with French police, but news that MEPs would be given preference over staff and assistants caused an uproar.
"I am proposing that either everyone leaves or no one leaves," said Edouard Martin, a French MEP.
The evacuation is especially tricky as the vast majority of EU parliament participants are from out of town, mainly Brussels, with thousands staying in hotels in the city.
When the news of the attack first broke, some responded angrily: "The lucky ones are back at their hotels," complained one official as she took an elevator back to her office.
But as the night grew longer, proceedings began to resemble a snow day from school, with the stranded killing time in good cheer.
"I'm safe but my heart and condolences are with the victims of this atrocity. Vive la France!," said British MEP Charles Tannock who was also under lockdown in the parliament building.
'Now we just wait'
In a bar usually reserved for MEPs, EU commissioners, powerful legislators and staffers huddled in groups waiting for developments.
Some watched football matches on tablets, while wine corks popped and hors d'oeuvres, scavenged from committee meetings, were passed around.
Belgian MEP Kathleen Van Brempt said no one knew anything more than what was being heard on the news.
"Tonight is our delegation Christmas dinner at a restaurant in the centre. Our first thought was for colleagues who had already made it to town, who are safe."
"Now we just wait," she said.
As bottles emptied and hours passed, staffers settled down where they could, with some assistants camping out in committee rooms with pizza and drink.
"We need some music in here, this is so boring!" shouted one attendee.
In the packed canteen, senior socialist MEP Udo Bullman did the rounds greeting colleagues.
"As far as we know our folks safe... Please stay cautious," he said on Twitter.
Green MEP Julia Reda was seen briefing colleagues on the latest parliamentary business as others surfed the internet for the latest developments in town.
"It's surreal," said Solange Helin-Villes, a spokeswoman for Social Democratic MEPS.
"We'll see how we're doing at 4am," she added.
Published : December 12, 2018
By : Agence France-Presse Strasbourg, France