Some 40 people were injured when the attacker drove a car into pedestrians on Westminster Bridge and crashed into railings outside the Palace of Westminster on Wednesday afternoon. A Metropolitan Police officer was one of those killed, as was the attacker.
Parliament Square and the surrounding area has been cordoned off since the incident, which began at about 2.40pm.
In a video message, Mr Khan said: "London is the greatest city in the world, and we stand together in the face of those who seek to harm us and our way of life.
"We always have and we always will. Londoners will never be cowed by terrorism."
"There will be additional armed and unarmed police officers on our streets from tonight in order to keep Londoners, and all those visiting our city, safe.
"I want to reassure all Londoners, and all our visitors, not to be alarmed."
The Met advises a three-step response for members of the public caught up in an attack like the one on Wednesday—Run, hide and tell, using the 999 service.Reuse content