Two men have been arrested over the murder of Pc David Phillips, who was mown down and killed by a driver in a stolen car.
The pair, aged 18 and 30 and from the Oxton and Wallasey areas of the Wirral, are being questioned in connection with the killing of the 34-year-old father of two in the early hours of Monday, Merseyside Police said.
The force said it also executed four warrants in the areas as part of the murder investigation into Pc Phillips' death.
The arrests came as a memorial fund was set up on Pc Phillips' memory, with Merseyside Police Federation chairman Peter Singleton saying the "entire police family" was in mourning for an officer killed while "simply doing his job".
The fund has already raised more than £18,000.
Flowers continue to be laid at the scene of the incident near the docks in Wallasey. However a nearby lamp post was also daubed with graffiti saying FTP, which stands for "f*** the police".
Detective Chief Superintendent Paul Richardson thanked the public for their support.
He said: "Since the investigation was launched yesterday we have been inundated with calls from members of the public and as a result we are following up a number of positive lines of inquiry.
"From the hundreds of supporting and caring comments on social media and media websites yesterday, it is obvious that there is overwhelming support for the police and that the public appreciate the work undertaken by officers like Pc Phillips to keep their communities safe.
"I would like to appeal to any members of the public who have any information which could help us with the investigation to come forward.
"You may not think that the information you have will mean that much, but for us that information could be the vital piece of the jigsaw we need and is essential in helping us to build up a picture in relation to what happened on the night."
Pc Phillips had been deployed to assist following a burglary at an estate agents in Birkenhead where a red Mitsubishi pick-up vehicle was stolen just after 1am on Monday morning.
Following a police pursuit at about 2.08am, Pc Phillips and a colleague had attempted to use a "stinger" device to stop the runaway Mitsubishi Challenger L200.
But the car mounted a central reservation and headed directly at the officer before driving off at speed. Pc Phillips was given CPR by colleagues at the scene but died a short time later at Arrowe Park Hospital on the Wirral.
A Home Office post-mortem examination said he died from internal injuries suffered in the impact with the vehicle, which was later found abandoned in Corbyn Street, Wallasey.
Police officers in forensic suits could be seen searching a three-storey mid-terrace house on Wheatland Lane, just yards from where the stolen pick-up was dumped in Wallasey.
Two officers stood guard outside while forensic investigators went in and out with equipment.
The Union Jack flag outside Wallasey Town Hall was being flown at half-mast as a steady stream of wellwishers came to leave messages of support and sign the book of condolences.
Rodger Chambers carried his daughter, Arabelle, one, accompanied by his pregnant wife Helen to pay their respects.
Mr Chambers, a construction worker from Wallasey, said: "A dad should be able to come home from work to his family.
"We have a lot of respect for our police, they do a good job. It's just tragic and disgusting and we thought we would come down and put something in the book. I hope the police get them."
Mrs Chambers, whose father is a police officer, added: "It has just hit home a little bit harder. It is upsetting.
"It is awful, he's 34 with two young children, same age and same as us."
Merseyside Police Federation Charitable Trust set up a Just Giving page to honour an officer "tragically and unlawfully killed whilst on duty".
Mr Singleton said: "The entire police family is in mourning at his death, and this memorial fund enables people to donate in Dave's memory. He was simply doing his job.
"We send our deepest condolences to Dave's wife Jen, and daughters Abigail and Sophie, as well as other family, colleagues and friends."
You can donate by visiting http://campaign.justgiving.com/charity/mpfct/dave or by texting DAVE01 £10 (or your amount) to 70070.