A PC has been injured after being "dragged along" by a van as he tried to speak to its occupants.
Merseyside Police said the officer was retrieving a cannabis-filled backpack which was left on Hewitt Avenue in St Helens at about 20:15 BST on Tuesday.
The force said he approached a nearby Ford Transit, which struck him and drove off at speed.
Two men, aged 26 and 30, from Rochdale were arrested on suspicion of causing grievous bodily harm with intent.
The 30-year-old was also held on suspicion of possession with intent to supply cocaine and cannabis.
A force spokesman said the officer was undergoing surgery after suffering a dislocated shoulder and fractured wrist and was in a stable condition in hospital.
'A great cost'
The PC had approached the white van in order to search it because its "occupants were acting suspiciously", he added.
Ass Ch Con Paul White said as the PC "has leant in, the driver has driven away so the officer has been dragged a short distance".
He thanked residents who came to the aid of the officer, saying: "We can't thank the public enough".
A short time after, it was reported that "two men were running from a van into fields" near the A580 East Lancashire Road and had "discarded a bag", police said.
The bag and a vehicle, which had been left on Ecclesfield Road and near to where the men were seen, were recovered for forensic examination.
Appealing for witnesses, Ch Insp Jim Wilde said his officers "fearlessly put themselves in harm's way" every day.
"They do so willingly, because they have joined the police service to protect people and keep Merseyside safe," he said.
"Sadly, this evening our officer's dedication has come at a great cost to himself.
"Thankfully, he is now receiving the care and treatment he needs, and everyone at Merseyside Police wishes him well."