Four people were killed and at least 20 injured after a car plowed into pedestrians and an attacker stabbed a policeman, in what police called a terrorist incident.

A woman assists an injured person after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

A woman assists an injured person after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

A woman lies injured after an incident on Westminster Bridge in London. REUTERS/Toby Melville

A woman lies injured after an incident on Westminster Bridge in London. REUTERS/Toby Melville

Paramedics treat an injured person after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

Paramedics treat an injured person after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

Injured people are assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

A woman lies injured after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

A woman lies injured after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

A man lies injured after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

A man lies injured after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

A woman ducks under a police tape after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

A woman ducks under a police tape after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

An armed police officer assists a woman after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

An armed police officer assists a woman after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

An injured person is assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

An injured person is assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain.  REUTERS/Toby Melville

Injured people are assisted after an incident on Westminster Bridge in London, Britain. REUTERS/Toby Melville

Police assist bystanders during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

Police assist bystanders during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

An air ambulance lands in Parliament Square during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

An air ambulance lands in Parliament Square during an incident on Westminster Bridge in London, Britain. REUTERS/Stefan Wermuth

Forensics investigators work at the scene after an attack on Westminster Bridge in London, Britain. REUTERS/Hannah McKay

Forensics investigators work at the scene after an attack on Westminster Bridge in London, Britain. REUTERS/Hannah McKay

Members of the emergency services take an injured person away on a stretcher after an incident on Westminster Bridge in London, Britain. REUTERS/Eddie Keogh

Members of the emergency services take an injured person away on a stretcher after an incident on Westminster Bridge in London, Britain.

Source: REUTERS