An American tourist has been killed and five others injured following a knife attack in central London on Wednesday.

The victims injured in the attack were from the United States, Australia, Israel and the UK, while the slain American tourist was identified as 60-year-old Darlene Horton from Florida.

London Metropolitan Police say they have arrested one suspect who has been identified as a 19-year-old Norwegian national of Somali descendant.

Security measures across the city have been stepped up in response to the attack.

"I just arrived a bit too late, when all the police was gathering, and it's pretty terrible what happened. It could be us, as it was in Munich, Nice or everywhere in Europe, Paris, so yes, it could be," said Samuel, who was working near the site of the attack at Russell Square.

Although police have said that mental illness could be the main cause of the attack, many Londoners still think the incident cannot be taken out of the context following the recent surge in terror attacks across Europe.

However, one resident urged for calm and insisted life will go on in the capital.

'I think it's very easy to just jump on the bandwagon and assume that it's a terrorist attack. And I think we have to accept that that might happen, you know, we are on severe alert. Londoners are very resilient and I think we just have to get on with it and live our life, because if we don't, then they have won. Life is for living, and I think most Londoners adopt that attitude," said Luce, London resident.

Some counter-terrorism experts warned that the authorities must pay close attention to extremist groups' attempts to radicalize mentally ill people in Europe.