Ten people have been arrested on suspicion of murdering a police officer killed while attending a reported burglary.
PC Andrew Harper, of Thames Valley Police, died at about 23:30 BST on Thursday in Berkshire.
The force said the 10 males arrested were aged between 13 and 30, and remain in custody.
Prime Minister Boris Johnson told the BBC the officer’s death was a “mindless and brutal crime”.
Police said PC Harper, who was from the Roads Policing Unit based in Abingdon, was “killed while performing his duties”, but have not confirmed any further details.
A cordon has been put in place at the scene near the A4 Bath Road between Reading and Newbury, at the crossroads near to Sulhamstead.
Mr Johnson said the officer’s death was “an absolute tragedy ” and “the most powerful reminder that police officers up and down the country put themselves at risk every single day to keep us safe”.
“My thoughts are with PC Harper’s family, friends and colleagues at this terrible time,” he added.
Deputy Chief Constable Jason Hogg said: “As a force we are devastated at the loss of our colleague and will all be supporting each other at this difficult time.”
Andy Fiddler, from the Thames Valley Police Federation, added: “This is totally devastating news.
“All our thoughts – and the thoughts of the entire police family across the UK – are with the family, friends and close colleagues of PC Andrew Harper who died last night.”
The Independent Office for Police Conduct (IOPC) said it had been made aware of the officer’s death, because of possible issues around a police pursuit, but is not investigating.
Home Secretary Priti Patel said: “PC Harper died in the line of duty protecting the public and his incredible bravery and extraordinary sacrifice will not be forgotten.”
Thames Valley Police said it was still trying to establish the circumstances in which PC Harper was killed.
Forensic investigators are at the scene, where two forensic tents have been put up.
Witnesses said officers were seen searching the inside of a grey BMW that was nearby.
How many police officers are killed on duty?
In the past 10 years at least 10 officers have been killed during an incident involving a criminal suspect.
The most recent case was the murder in 2017 of PC Keith Palmer, who was stabbed during the Westminster terror attack in 2017.
In 2012, Greater Manchester PCs Fiona Bone and Nicola Hughes were killed in a grenade and firearm attack, while investigating a hoax burglary call.
A number of the other cases have involved officers being struck by vehicles driven by suspects.
Violence against officers is a common occurrence. In the year to March, there were 30,977 assaults on police officers and a third of these resulted in injury.
The attack on PC Harper is the third time in recent weeks an officer has been seriously hurt in the line of duty.
Metropolitan police constable Stuart Outten was stabbed in the head on 8 August.
Days later a 42-year-old West Midlands Police officer suffered serious head and pelvic injuries when he was run over with his own vehicle.
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