London attack: Major parts of city in lock down as extra police deployed across capital

Extra police are being deployed across London and many of the city's major tourist areas are in lockdown following a terrorist incident outside parliament.

Commander BJ Harrington, addressing media outside Scotland Yard, said police were investigating varying eyewitness accounts including a person in the River Thames after a car appeared to mow down a crowd on the Westminster before an armed man attacked officers at Westminster Palace.

"Although we remain open-minded to the motive, a full counter-terrorism (operation) is underway," Commander Harrington said.

Metropolitan police deputy commissioner Craig Mackey was at the scene and is being treated as a significant witness, Commander Harrington said.

The public is being told to avoid Parliament Square, Whitehall, Westminster Bridge, Lambeth Bridge, Victoria Street up to the intersection with Broadway, and the Victoria Embankment up to Embankment Station.

Anyone who sees anything suspicious should call 999 immediately, Commander Harrington said.

He appealed for witnesses with photographs or footage of the incident to hand it over to police.

Policing in the capital is being reviewed and extra officers will be on duty.

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