A police officer and father of two young children was mown down and killed as he tried to stop a stolen pick-up truck which was being pursued after a burglary.
Pc David Phillips "did not stand a chance" when the vehicle drove directly at him in Wallasey, Wirral, his chief constable said.
Sir Jon Murphy admitted police had "no idea" who was responsible for the tragedy, but warned the culprits: "We will find you."
Pc Phillips was standing on a central reservation when the Mitsubishi mounted the kerb and hit him at speed before driving off.
The stolen vehicle was found abandoned nearby a short time later, Merseyside Police said.
Sir Jon said the 34-year-old officer was with a young colleague who tried to stop the stolen vehicle using a stinger device to puncture its tyres.
"The Mitsubishi vehicle mounted the central reservation, drove directly at the officers down the central reservation and hit Constable Phillips. He did not stand a chance," he said.
Sir Jon appealed for those responsible to do the "sensible thing".
He admitted the evidence police have currently is the abandoned car and "nothing else".
He emphasised the appeal for information, adding: "We have got no idea who has done this."
Pc Phillips was married and he and his wife, Jen, had two girls, Abigail, aged seven, and three-year-old Sophie.
He joined Merseyside Police in 2006 and was "popular and held in high regard" by his colleagues.
Sir Jon said Pc Phillips is the first Merseyside Police officer to be killed on duty as a result of a criminal act since 1981 and paid tribute to the bravery of his officers.
"David's death serves as a reminder of the risks that the men and women of this force and the other forces in the UK face, in serving the public.
"They come to work, day in day out, knowing the risks they all face.
"But their wives, their husbands, their loved ones and their families expect them to come home at the end of their watch.
"Sadly, David did not and his daughters will have to grow up without their father," he said.
Merseyside police and crime commissioner Jane Kennedy's voice broke with emotion as she spoke to dozens of TV crews, reporters and photographers at the press conference at the force HQ in Liverpool.
She said: "Police officers are on the front line every single day protecting our communities.
"It's a dark day when you lose one of our own and it's a painful reminder of the risks police officers face to keep us all safe."
Peter Singleton, chairman of Merseyside Police Federation, said Pc Phillips' colleagues are "understandably devastated", adding: "When any officer is killed on duty it is a tragedy and in Dave's case it is possibly more so. His young children have been robbed of a father, his wife Jen, deprived of a husband and his parents, their son.
"His colleagues and friends have (lost) a respected and truly exceptional officer.
"He was one of the best and will always be missed."
A police presence remains outside Oxton Estates in Woodhouse Road, Birkenhead, where the Mitsubishi was stolen.
Residents said the car was usually parked outside the sales and letting shop and had stickers advertising the business.
One neighbour who did not want to be named said "it stuck out like a sore thumb".
Maria Driver, 55, who saw the news on Facebook, said: "It's not unusual around here, the last 12 months has been absolutely horrendous for car thefts, motorbike thefts and burglaries. The alarms were going off the other night about 6:30pm.
"It's so sad, his poor family. It's horrible."
Anyone with information is asked to call Matrix serious organised crime unit on 0800 230 0600 or Crimestoppers on 0800 555 111.