Update: King's Cross Tube station is being evacuated because of a suspected fire, not a suspicious package.
Hundreds of commuters were told to get out of the station, with witnesses saying alarms were going off throughout the station at around 11:30am.
One witness said staff had told him that a "suspicious package" had been found by police. However this was quickly dispelled.
Alarms going off throughout Kings Cross Underground Station due to a 'reported emergency'.— David Hickling (@DavidsHickling) March 23, 2016
Definite sense of urgency to get out when the tube station gets evacuated for an 'emergency' at the moment #KingsCross— Tara Hounslea (@Tara_Hounslea) March 23, 2016
But it's clear that people are on edge, with the UK's security being stepped up in the wake of yesterday's terror attacks in Brussels.
Additional officers have been mobilised throughout London to carry out "highly visible patrols at key locations around the capital including the transport network".
The officers were being deployed as part of "reassurance measures", and not because of any specific intelligence, according to Mark Rowley, national lead for the Metropolitan Police's counter-terrorism arm.
More to follow....