Labour MP Jo Cox has died after being shot and stabbed in the street outside her constituency advice surgery.
The mother of two was attacked by a man reportedly shouting "Britain first" at lunchtime in Birstall, West Yorkshire.
Temporary chief constable for West Yorkshire Dee Collins told a news conference: "I am now very sad to have to report that she has died as a result of her injuries."
Labour leader Jeremy Corbyn said the country would be "in shock at the horrific murder" of the MP, who was a "much loved colleague".
The alleged gunman has been named locally as Tommy Mair, 52, who neighbours in Birstall have described as "a loner".
Ms Collins expressed her deepest condolences to the family and friends of Ms Cox, who had two children aged three and five.
She said: "Jo was attacked by a man who inflicted serious and sadly, ultimately fatal injuries. Subsequently there was a further attack on a 77-year-old man nearby who has sustained injuries that are non-life threatening."
She added: "This is a very significant investigation with a large number of witnesses being spoken to at this time.
"It's a large and significant crime scene and a large police presence with a full investigation is under way to establish the motive for this attack."
Ms Collins continued: "Shortly afterwards a man was arrested nearby by local uniformed police officers. Weapons, including a firearm, have also been recovered.
"At 1.48pm Jo Cox was pronounced deceased by a doctor who was working with a paramedic crew that were attending to her serious injuries.
"This is a very significant investigation with large numbers of witnesses that have been spoken to by police at this time. There is a large and significant crime scene and there is a large police presence in the area.
"A full investigation is under way to establish the motive for this attack."