Police have advised individuals to hide inside structures, while officers react to reports of an episode at London's Oxford Circus Tube station.
English Transport Police said officers were at the scene and cautioned individuals to maintain a strategic distance from the zone in London's West End.
"Oxford Circus Station is shut while we research a client occurrence," Transport for London said.
BBC journalist Helen Bushby said she had seen a "mass charge" of individuals fleeing from the station.
Individuals were "shouting and crying", she said.