A police officer was killed when he was "dragged along by a vehicle" while attending a reported burglary.
PC Andrew Harper, 28, who got married four weeks ago, died at about 23:30 BST on Thursday in Berkshire.
Thames Valley Police said 10 males, aged between 13 and 30, have been arrested on suspicion of murder and remain in custody at various stations.
Prime Minister Boris Johnson told the BBC the officer's death was a "mindless and brutal crime".
Thames Valley Police Chief Constable John Campbell said the death of PC Harper, an Abingdon-based roads policing officer, had left the entire force "shocked and saddened".
He told a press conference that although the cause of PC Harper's death had not yet been confirmed, he was out of his police car when he was "dragged along by a vehicle".
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.
Mr Campbell said PC Harper was a "highly regarded, popular member of the team" and his death was a "significant loss" to the force and also to his colleagues and friends.
Members of PC Harper's family paid tributes on social media to the "loveliest person that you will ever meet".
Maureen Shrimpton wrote: "Just a really horrible day. Our grandson Andrew was killed last night while doing his police work.
"So proud of him. Our love goes to his lovely wife, mum and dad, brother and all of his family and friends."
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 exact circumstances in which PC Harper was killed.
The force said PC Harper, who became a regular officer in 2011 after joining as a special constable a year earlier, had attended the reported break-in with a crewmate.
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.
The force said it would fly its flags at half-mast "in honour and memory of Andrew".
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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