In a major show of strength by the Metropolitan Police, two of the force's 13 Jankel Guardians were parked in Parliament Square.
The vehicles, built in Surrey, are considered "military spec" and cost £128,000.
They were bought by the Met for use in several units including riot control and firearms, and are aimed at helping combat terrorism, gun violence and hijackings.
Their appearance in the capital came as the number of armed police in London and other major cities across the UK was stepped up.
Scotland Yard's anti-terror chief Mark Rowley said: "The police service will sustain an enhanced armed and unarmed presence over the next few days.
“London, and the UK, are open for business, and we are out there in greater numbers to make sure that the public see a highly visible presence to help reassure them as they go about their daily lives.”
Four people were killed by Khalid Masood before the terrorist was shot dead by police in Wednesday's horrific attack.