Commuters were evacuated from platforms at one of the UK's busiest railway stations while armed officers hunted for a suspected drug offender.
British Transport Police cleared a number of platforms and forced some trains to a standstill at King's Cross Station in London shortly after 3pm while officers waited for a train to arrive.
Armed officers boarded the carriage and arrested a 40-year-old man on suspicion of drugs offences, BTP said.
Eyewitness said armed officers surrounded a man while a "sea of people" were ushered away.
A BTP spokesman said: "Armed BTP officers attended the mainline platforms at King's Cross rail station shortly after 3pm today to meet incoming trains to that location.
"Some platforms were closed while this activity took place.
"At approximately 4.15pm, a 40-year-old man was arrested on suspicion of possession with intent to supply of a Class B drug. Enquires are ongoing."
Eyewitness Chris Gibbons tweeted: "Sea of people stuck at #KingsCross due to a "security incident". Platforms closed and lots of armed police!"
Jack Marshall added: "Crazy scenes, held on the train at #KingsCross then suddenly armed police everywhere surrounding a guy."
National Rail said all platforms have now reopened, however there are some short train delays as the service returns to normal.
King's Cross is one of the UK's busiest stations. Last year nearly 30 million passengers passed through its 12 platforms, according to National Rail data.