Updates from the last hour:
French Prime Minister Bernard Cazeneuve has confirmed that a group of French students have been injured in the incident, with unconfirmed reports from French media saying that three have been hospitalised, with no details as of yet on their condition.
According to Andrew Sparrow with the Guardian, the journalists who were at the press gallery are now being held in Westminster Hall. Police are searching the palace is it is believed a suspect is on the loose.
Also from the Guardian, it has been reported that another police press conference is due following reports that a police officer was killed in the incident.
Theresa May is expected to chair an emergency meeting of COBRA due to the incident.
More updates soon.