Kings Cross station was evacuated by armed police as a man was detained on suspicion of drug offences.
Armed British Transport Police (BTP) descended on mainline platforms shortly after 3pm to meet a train coming into the London station, the force said in a statement.
Officers arrested a 40-year-old man on suspicion of possession with intent to supply a Class B drug over an hour later, at 4.15pm.
As rush hour was set to start in the capital, closed platforms were evacuated as officers made inquiries. Platforms have since re-opened, BTP tweeted, thanking passengers for their co-operation.
All platforms at #KingsCross now starting to reopen – thank you to everyone at the station for your patience and co-operation.— BTP (@BTP) August 13, 2015
Trains travelling to and from from the station were delayed by up to 30 minutes, according to the National Rail website.
By 5pm, trains were running normally once again, according to the rail organisation.
Passengers at the station tweeted updates as the scene unfolded.
One described seeing a man being led away by armed police at the station.
Man in handcuffs being led away by armed police at King's Cross.— Gav (@GTRussell) August 13, 2015
Kings Cross closed. Police in front of entrances.— Stuart Dinsey (@Sdinsey1) August 13, 2015
#kingscross platforms 1 to 8 closed for police search— Simon Allcock (@SimonAllcock) August 13, 2015
Armed guards at a closed #kingscross - yikes— Rebecka Steven (@BeckaSteven) August 13, 2015