British Transport Police (BTP) have detained a 40 year-old man on suspicion of intent to supply a Class B drug at King's Cross Station, after hundreds of passengers were evacuated just before the beginning of the evening rush hour.
On Twitter, users reported the station being "full of armed police". Others said platforms 1-8 were closed, with staff removed from all trains.
National Rail tweeted that emergency services were dealing with "an incident", delaying journeys by up to 15 minutes to and from the station.
A spokesperson told City A.M. British Transport Police (BTP) had been called to meet a train coming from the North "due to an incident on board that train".
The BTP said some platforms would remain closed while it carried out enquiries.
In a statement, they 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. Enquiries are ongoing.
"The operation is now coming to an end and the platforms are being re-opened."