man assault police officer four-foot sword outside queen elizabeth buckingham palace residence shout allahu akbar god great question counter-terrorism police saturday two unarm officer suffer slight cut detain man drive police van friday even take sword front passenger foot-well car london metropolitan police say early say man plan say commander dean haydon head meet counter terrorism command believe man act alone look suspect stage say right investigate terrorist incident time europe high alert follow string militant attack include four year britain kill 36 people country threat level remain severe mean attack highly likely members royal family present palace magnet tourists britain capital peak august holiday weekend want thank officer act quickly bravely protect public last night demonstrate dedication professionalism police prime minister theresa may say message twitter suspect initially arrest suspicion grievous bodily harm assault police arrest britain terrorism act police say investigate 26-year-old man luton area ethnically diverse town 35 miles 55 km north london police carry investigations link militant attack include one early year london westminster bridge partner saw sword ... well policeman blood look like hand chest injure police officer hand walk away say unnamed witness quote time newspaper say tourists run away scene something happen people run away sure people already scar saw policeman pull man car witness say suspect treat london hospital minor injuries report injuries timely reminder threat terrorism uk remain severe haydon add police together security service everything protect public already enhance police plan bank holiday weekend keep public safe man sword injure police outside uk queen 's palace