A police officer remains in hospital after he was dragged along the road while attempting to search a van.
The Merseyside police constable is undergoing surgery for a dislocated shoulder and also suffered a fractured wrist after he was called to Hewitt Avenue, St Helens, on Tuesday evening, a force spokesman said.
Two men are being questioned following the incident.
Police said officers were called to the scene at 8.05pm by a member of the public who reported a backpack suspected to contain cannabis.
Once there, they retrieved the bag and noticed a white Ford Transit van with occupants acting suspiciously nearby.
Assistant Chief Constable Paul White said the officers approached the vehicle to search it.
He said: “One of the officers has leant in and the driver has driven away so the officer has been dragged a short distance.”
He said members of the public came to the aid of the constable and shortly after police received a report of two males running from a van into fields near Ecclesfield Road.
Two men from Rochdale, aged 26 and 30, were arrested on suspicion of causing grievous bodily harm with intent and remain in custody.
The 30-year-old was also arrested on suspicion of possession with intent to supply cocaine and cannabis, police said.
Mr White said: “Officers do understand the risks of the job but that never makes it acceptable for our officers to have to face this level of violence.
“It is totally unacceptable and could have a massive impact both physically and psychologically and on colleagues and family.
“Potentially the injuries could have been a lot worse, so in that respect he is reasonably fortunate but he still has very serious injuries.”
He thanked residents who immediately came to the aid of the injured officer.
“We can’t thank the public enough,” he added.
Mr White said CCTV was being examined and appealed for anyone with information or dashcam footage to contact police.
Merseyside police and crime commissioner Emily Spurrell said: “Officers serve every day to protect the public.
“Violence against them is utterly unacceptable.
“My thoughts are with this officer and his family and I wish him a very speedy and full recovery.”
Anyone with information is asked to contact the force via Twitter direct message @MerPolCC or at ‘Merseyside Police Contact Centre’ on Facebook quoting reference 22000339976.
Alternatively, they can call Crimestoppers anonymously on 0800 555 111.