The man was known to the police and was stopped and searched as part of an ongoing operation. He was found to have a rucksack full of knives. He was arrested for possession of an offensive weapon and commission, preparation and instigation of acts of terrorism.
Meanwhile, a woman has been shot by police during an anti-terrorism raid elsewhere in London. Three people - a man, a woman and a 16 year old male - were arrested on suspicion of preparing to to carry out a terrorist act and another woman was arrested in Kent as part of the same operation. Police say these raids were unrelated to the arrest in Whitehall.
The police have yet to confirm that the suspects are all mentally ill.