Metropolitan Police Commissioner Cressida Dick praised officers' courage as they leapt into action following the "ghastly" incident on Tuesday.
Since the Westminster Bridge attack in March last year, Ms Dick said there had been an increase in the number of armed officers and barriers in place to protect the capital's political heart.
Discussions between parliamentary authorities, security agencies, the police and London's mayor will "no doubt" take place about whether the site should be further pedestrianised, she said.
But the commissioner, speaking ahead of a series of unrelated dawn raids that took place in Lewisham, south east London, on Wednesday said: "As with anything there is a balance to be drawn."
She added: "We are not going to give in, we are not going to just change our lifestyle, but it is important we take reasonable measures, as I think we have being doing over the last several months, to try and make sure that the most iconic sites, including those in central London, are well protected and should something happen there, that the police are able to respond very quickly with armed officers, which is exactly what we saw yesterday."