Five people died and at least 40 were injured after an attacker drove a car along a pavement in Westminster, stabbed a policeman and was shot dead by police in the grounds of Parliament.
PM Theresa May said the attack was "sick and depraved" and that it struck at values of liberty, democracy and freedom of speech.
"I received the news about the terrorist attack in the very heart of London with great sadness. This is yet another indicator that we must show determination in the fight against terrorism and secure safety for all citizens," Plenkovic said in his note to May.
The attack unfolded at about 14.40 GMT when a single attacker drove a grey Hyundai i40 along a pavement over Westminster Bridge, near the Houses of Parliament in central London, killing at least two people and injuring many more.
The car then crashed into railings outside the Houses of Parliament. The attacker, armed with a knife, ran to Parliament where he was confronted by the police. A police officer who was not armed was then stabbed and killed. The attacker was shot dead by armed officers.
Text: Hina