Prime Minister Theresa May has told MPs: "We will never waver in the face of terrorism'', according to BBC.
''This was an attack on free people everywhere'', added also she.
Paying tribute to Pc Keith Palmer, who died after being stabbed, she said: "He was every inch a hero and his actions will never be forgotten."
Delivering a Commons statement she said Wednesday's attacker was British born and known to police and MI5. The attack was related to "Islamist ideology".
Labour leader Jeremy Corbyn described the attack as "an appalling atrocity".
Four people died, including the attacker, with 40 more injured.