London police have declared the incident outside the Houses of Parliament a "terrorist" attack conducted by only one attacker.
Isis supporters have cheered the attack on Westminster, suggesting it was "revenge" for the UK's airstrikes on the terror group in Syria and Iraq.
Followers on pro-Islamic State of Iraq and the Levant channels on the social media service Telegram posted messages applauding the knife-wielding suspect and called the attack"blessed".
One said the UK was paying "blood for blood" for its involvement in the US-led coalition's campaign against the jihadists.
"Our battle on your land is only just beginning," read one poster next to a photoshopped image of Big Ben being blown up.
2. Pro-ISIS channels are rejoicing at the bloodshed including one that has created a poster to go with the violence. (Thanks @sweattloafer) pic.twitter.com/jSb6xzDgy0
At the time of writing, however, there was no official claim of responsibility. Attacks organised and inspired by Isil are usually not claimed within the first 24 hours.
Motivations of the attacker are not yet known, but the use of a vehicle as well as a knife are consistent with instructions for attacks issued by Isil and are similar to the group's attack on a Christmas market in Berlin and in Nice, which left more than 80 dead.
Abu Mohammad al-Adnani, the late Isis spokesman, instructed European followers to carry out attacks at home in an audio message released after the declaration of its so-called caliphate.
"If you are not able to find an IED or a bullet, then single out the disbelieving American, Frenchman, or any of their allies. Smash his head with a rock, or slaughter him with a knife, or run him over with your car," he said.
Isil has been explicitly threatening attacks on Britain for the last 18 months.
At end of a propaganda video claiming the Paris attacks in November 2015, Isis singled out the UK as its next target.
The UK is second only to France in the number of fighters it has sent from Europe to Syria and Iraq. But until now Britain has been spared the deadly attacks seen by countries like France, Belgium and Germany.
Harry Sarfo, a German Isil fighter educated in England who is serving a sentence for terror offences, said the jihadists had trouble recruiting people for attacks in the UK.
Image 1 of 16: People stand near a crashed car and an injured person lying on the ground, right, on Bridge Street near the Houses of Parliament in London. Photo / AP
He told the New York Times in an interview last year that although the group was well set up in some European countries, it needed more attackers in Germany and Britain, in particular.
