London attack

Donald Trump Jr tweeted an article written last year, in which Sadiq Khan said terror vigilance had become "part and parcel" of life in a global city.

"You have to be kidding me?!" said Mr Trump, immediately sparking accusations that he was exploiting the tragedy and misrepresenting the mayor's point.

Dozens were hurt in the London attack.

Scores more people were hurt when the attacker plowed his car through a crowd of people on a bridge over the River Thames, got out of his vehicle and fatally stabbed police officer Keith Palmer, a 15-year-veteran who was unarmed, on the grounds of Parliament.

An armed officer shot the assailant dead.

Mark Rowley, Britain's most senior counterterror police officer, told reporters late Wednesday that authorities believe they know the attacker's identity but are not releasing it, and are operating on the assumption the incident was "Islamist-related terrorism."

The assault, which came on the one-year anniversary of the Isis bomb attacks in Brussels, brought carnage to Westminster, the heart of political life in Britain.

Among the dead were a police officer and the assailant, police said, announcing a "full counter-terrorism investigation." A woman was pulled from the River Thames alive, but with serious injuries.

Police and ambulance crews were called at 22:33 BST on Wednesday to reports of a man in possession of a knife and injuring people.

Up to six injured people were found at the scene; one woman was pronounced dead a short time later.

The man was arrested at 22:39; a taser was discharged by one of the arresting officers.

Terrorism is one possible motive being explored, the Metropolitan Police say.