He said policing in London was constantly evolving as new threats emerged. Some of the roads around the Houses of Parliament remain closed, and New Palace Yard, where PC Keith Palmer was stabbed, is still being treated as a crime scene, and forensic officers are currently at work on the site. Annual leave for police has been cancelled and officers are working extended shift patterns, up from eight hours to about 12 hours and sometimes more. A service is now taking place in front of Scotland Yard, in front of the flame that burns as a tribute to all dead Metropolitan police officers. At Westminster a minute’s silence is being observed in honor of the victims of yesterday’s attack. The mayor of London, Sadiq Khan, has issued an open invitation to all Londoners and visitors to the capital to a candlelight vigil in Trafalgar Square from 6pm this evening.” London is the greatest city in the world. We will never be cowed by terrorism. We stand together, in the face of those who seek to harm us and destroy our way of life. We always have, and we always will,” he said in a statement.