For most of its history, Downing Street was accessible to the public. There was some security at Number 10 but it was minimal: a police officer standing guard. The front door has no keyhole on the outside. A second officer is on duty in the entrance hall to open it for the Prime Minister.
During the premiership of Margaret Thatcher gates were installed at both ends of the street due to terrorist threats. On 7 February 1991, a mortar shell (минометный снаряд) was launched at Number 10 from a van parked in Whitehall. It exploded in the back garden, while Prime Minister John Major was holding a Cabinet meeting. Because of this attack, heavier security measures were imposed including retractable (выдвижной) barriers barring access and a guardhouse standing at the gated entrance.