Ten people have been arrested on suspicion of murdering Pc Andrew Harper.
The 28-year-old had been dragged along by a vehicle and died just four weeks after his wedding, Thames Valley Police Chief Constable John Campbell said.
Thames Valley Mr Campbell said it was a “terrible day” for the force.
At a press conference, he said: “We are all shocked and saddened by the death of our colleague Pc Andrew Harper.”
He added: “Tragically Andrew was attending a reported burglary along with a crew mate and was killed at the scene whilst performing his duties.”
Mr Campbell said: “Andrew was a roads policing officer who was based at Abingdon police station.
“He joined Thames Valley Police as a special constable in 2010 and became a regular police officer in 2011.
“Andrew was 28 years old. Our thoughts are with Andrew’s friends and family who are being supported obviously by specially trained family liaison officers.
“And to add further angst to what are terrible circumstances, Andrew was only married four weeks ago.”
Pictures show Pc Harper and his new wife Lissie celebrating their wedding at the award-winning stately venue Ardington House, a Georgian manor in Oxfordshire set in gardens and parkland.
Relatives described the day as a “dream wedding”.
Prime Minister Boris Johnson said he was “deeply shocked” and “appalled” at the news Pc Andrew Harper had died after a “serious incident” around 11:30pm on Thursday near the A4, Bath Road, between Reading and Newbury.
Thames Valley Police said ten boys and men aged between 13 and 30 have been arrested on suspicion of murder and remain in custody at various police stations in the force area.
The incident occurred at the crossroads of Ufton Lane and Lambdens Hill, near to the village of Sulhamstead, police said.
Labour leader Jeremy Corbyn said he was “totally shocked” by the death and said it highlighted the need for more police officers on the streets.
Police said they were still establishing the circumstances in which Pc Harper, from the Roads Policing Proactive Unit based at Abingdon Police Station, was killed while carrying out his duties.
Officers remain at the scene of the incident, which is cordoned off while evidence is gathered.
On Thursday afternoon forensic investigators could be seen gathering items that lay on the road and taking photographs. Two white tents have also been erected.
Police were also seen searching the inside of a grey BMW that was at the scene, according to witnesses.
Pictures of the scene show a blood trail on the road leading across the junction.
Police officers laid two bunches of flowers inside the cordon, while other officers guarded the perimeter.
Ufton Lane and Lambdens Hill remain closed but the A4, Bath Road, has reopened, police said.
Pc Harper is the first officer to be killed on duty since March 2017, when unarmed Pc Keith Palmer was stabbed by Khalid Masood during the Westminster Bridge terror attack.
The incident comes after a police constable was run over by a suspected car thief in Birmingham last week.
The married 42-year-old traffic officer from West Midlands Police is facing “potentially life-changing” injuries, a senior officer said.
This came just days after Metropolitan Police constable Stuart Outten, 28, was left with head and hand injuries after challenging a motor offences suspect allegedly armed with a machete in Leyton, east London.
Mr Johnson’s comments followed his pledge to hire 20,000 more police officers and an announcement of a range of investments to boost the criminal justice system. He also vowed to see violent offenders locked up for longer as a result of an “urgent review” of sentencing laws.
Andy Fiddler, from Thames Valley Police Federation, said the death of Pc Harper was “totally devastating”, adding: “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.”
Deputy Chief Constable Jason Hogg said: “Our deepest thoughts at this time are of course with Andrew’s friends and family, who are being supported by specially trained family liaison officers.
“Our thoughts also extend to all of Andrew’s colleagues within Thames Valley Police.
“As a force we are devastated at the loss of our colleague and will all be supporting each other at this difficult time.”
Senior investigating officer Detective Chief Inspector Stuart Blaik, of the Thames Valley Police Major Crime Unit, said: “This is a tragic incident, and I would like to reassure the public as well as our officers and staff that we have launched a full and thorough investigation to establish what has happened.
“We have made a total of ten arrests in connection with this incident, and those arrested are currently in police custody. Those arrested are all male and aged between 13 and 30.
“I would appeal to anyone who witnessed this incident to get in touch. If you have any information which you think could be at all relevant to our investigation, no matter how insignificant it may seem, please leave details on our website or call 101.”