The attack took place on the first anniversary of attacks that killed 32 people in Brussels.
India condemned the terror strike in Westminster near the UK Parliament, saying there is no place for terrorism in democracies and civilised societies.
"India strongly condemns Westminster terrorist incident & condoles loss of life. No place in democracies & civilised societies for terrorism (sic)," spokesperson, Ministry of External Affairs, Gopal Baglay tweeted.
Minister of External Affairs Minister Sushma Swaraj said she was keeping a tab on the developments and tweeted, "I am in constant touch with Indian High Commission in London. There is no Indian casualty reported so far."
Rajasthan Chief Minister Vasundhara Raje also tweeted, "Strongly condemn the attack on #UKParliament in Westminster, London. My prayers are with the people of UK in this hour of grief."
Four people were killed and at least 20 injured in London on Wednesday after a car rammed pedestrians and an attacker stabbed a policeman close to the British parliament, in what police called a terrorist incident.
Prime Minister Theresa May was safe after the incident, a spokesman for her office said. He declined to say where May was when the attack took place.
Three French schoolchildren aged 15 or 16 were among those injured in the attack, French officials said.
The attack took place on the first anniversary of attacks by terrorists that killed 32 people in Brussels.
The House of Commons, which was in session at the time, was immediately suspended and lawmakers were asked to stay inside.
Indian High Commission in the UK said, "Any Indians injured during the Westminster incident may reach Public Response Unit of HCI ASAP: info.london@hcilondon.in; 02086295950."
(With inputs from PTI and Reuters)