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By Alastair Jamieson

LONDON — Armed police swarmed one of London’s busiest stations on Thursday to arrest a passenger in an operation that forced the evacuation of thousands of commuters.

British Transport Police officers were called to meet a train at the capital’s King’s Cross Station which had arrived from the northern England city of Leeds.

“As we arrived we saw armed police and they told us we had to get off the platform as soon as possible,” said eyewitness Gemma Trippas, 34, from London, who was on an earlier train.

Sea of people stuck at #KingsCross due to a "security incident". Platforms closed and lots of armed police!Chris Gibbons

“Everyone else was being kept behind the ticket barriers and nobody was being allowed in.”

The suspect was detained “following an incident on the train,” which arrived at 3.58 p.m. local time (10.58 p.m. ET), said a spokesman for station operator, Network Rail.

British Transport Police said in a statement that armed officers were on platforms to meet incoming trains and that some platforms were closed during the operation.

"A 40-year-old man was arrested on suspicion of possession with intent to supply a Class B drug," transport police added. "The operation is now coming to an end and the platforms are being re-opened.”

Class B drugs in the U.K. include cannabis and amphetamines.

Incidents at King's Cross can hit a nerve in the U.K. The station was among those affected in the July 7, 2005 suicide attacks on London's transit network which left 52 commuters dead.

Cassandra Vinograd contributed.