Four killed in attack on symbol of British democracy

London : Four people were killed and 40 injured after being run over and stabbed in a lightning attack at the gates of British democracy attributed by police to "Islamist-related terrorism".

The attack unfolded on Wednesday across Westminster Bridge in the shadow of Big Ben, a towering landmark that draws tourists by the millions and stands over Britain's Houses of Parliament -- the very image of London.

The attacker's car struck pedestrians on the bridge before crashing into the railings surrounding the heavily guarded Houses of Parliament, sowing first shock then panic in the seat of British power.

The assailant then ran through the gates brandishing a knife and stabbed a 48-year-old policeman to death before being shot dead by another officer.

Prime Minister Theresa May described the attack as "sick and depraved" in a defiant address in which she affirmed parliament would meet as normal on Thursday.

Standing outside her Downing Street residence after an emergency cabinet meeting, May said Britain's alert level would be kept unchanged.

"We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart," said May, who was dressed in black.

The prime minister was in parliament at the time of the attack and was seen being ushered away in a silver car as what sounded like gunfire rang out, British media reported.