Passengers ran for their lives after hearing a blast and seeing smoke while waiting in the departure area of the station shortly after 8pm on Monday.
Witness Haxie Meyers-Belkin told the Standard: "I was sat near the escalator leading to the platform when I noticed people screaming, dropping their cases.
"It was a mass panic of people running in all directions. People were trying to find corners to hide.
"The smoke was pretty acrid, so I thought something was amiss. But after 25 seconds it became clear it wasn't a bomb attack and it subsided pretty quickly."
#Eurostar update: 'We did have an #explosion in the terminal. This was due to a phone exploding in a passenger's bag. Some passengers were evacuated from the terminal'. A relief, certainly, but extraordinary that it took @EurostarUK so long to reassure passengers.— Haxie Meyers-Belkin (@haxieMB)February 11, 2019
The 8.10pm Eurostar service left as scheduled, though fellow witness Katy Lee said that amid the confusion, it departed without "200" disgruntled Parisians.
All calm at St Pancras now, just around 200 irate Parisians incredulous that the train left without us. N’importe quoi !
A staff member told me it was a Samsung phone that exploded.— Katy Lee (@kjalee)February 11, 2019
Posting on Twitter, she also said some people where left shaking and in tears following the scare.
A man, apparently a staff member, was seen photographing the debris of the device on the floor.
British Transport Police and Eurostar have been contacted for comment.