The US president's four-day tour will feature talks with British PM Theresa May, tea with the Queen, and mass protests including a giant Trump baby blimp being flown over Westminster.
An attacker drove a car along a pavement in Westminster, stabbed a policeman and was shot dead by police in the grounds of Parliament.
The police is investigating whether it was 'accidental or deliberate'.
Police is expecting the death toll to rise.
Terrorism struck at the heart of London after a vehicle veered off the road and mowed down pedestrians on London Bridge and witnesses described men with large knives stabbing passersby at nearby Borough Market.