Barnfield said a man on the platform was restrained by other visitors until police arrived. She said he “just stood there and was quite calm.”
Police say they don’t believe the suspect and victim knew each other.
“This was a truly shocking incident, and people will understandably be searching for answers,” said Detective Chief Superintendent John Massey of the Metropolitan Police. “At the moment, this is being treated as an isolated event with no distinct or apparent motive.”
Massey said police wanted to speak to anyone who “witnessed a male whose behavior seemed out of place, suspicious or worrying, in the hour or two before the incident in or near the gallery.”
Tate Modern, Britain’s leading gallery of modern art, sits on the south bank of the River Thames and was visited by almost 6 million people last year.
The 10th-floor terrace is part of a pyramid-shaped extension that opened in 2016 and offers panoramic views over London.