The officer was attacked during a night of disorder in the town centre.
The local police officer was on duty and talking to members of the public in Barras Street at around 9.30pm on Saturday when the incident happened, said Devon and Cornwall Police.
“It was reported that the officer was approached from behind and assaulted with a bottle to the head before being pushed to the floor where he was further assaulted,” police said.
“He was taken to Derriford Hospital where he was treated and has since been discharged.”
Two men aged 32 and 29 and a 34-year-old woman were arrested on suspicion of assault causing grievous bodily harm with intent, and were later released on police bail until mid-January.
Liskeard was busier than usual on Saturday evening: groups of friends and work colleagues were out for Christmas dos, and a number of people had come into town earlier in the afternoon to attend a boxing match.
One person who’d been at the boxing said it had been a “well-planned family event”, and another local person said: “the boxing event was really well run. We were out by sevenish and had dinner in Spoons. There were lots of people around but it was a calm atmosphere and no sign of trouble at 8.30pm when I went home .”
Landlord Alex McKay said that his pub, the Albion, had been busy with people who’d come to see a live band. After about 8pm, the pub was still open but with a further influx of people, he shut the doors so as to be able to regulate who came in.
By 9.30pm, there was building unrest in the town centre. A property had been vandalised, and several police cars had arrived.
The focus of the disorder was on the corner of Dean Street and Barras Street, and the assault took place close to the former NatWest building.
This area of the town centre was cordoned off by police, and the Wetherspoon pub shut its doors to the public for a short period while police officers dealt with the situation. Later, the pub re-opened, but reduced the amount of people being let in.
One person said: “We watched from our house round the corner and people were being idiots for hours. Loads of hard men trying to go back and start new fights.”
East Cornwall Chief Inspector Rupert Engley said: "I recognise that the incident over the weekend will be unsettling for those hearing about it, but these situations are rare and this was an isolated incident, overall crime levels remain low across the sector and across Cornwall."
In terms of policing, the sector had "resourced as it normally would do for expected demand over the Christmas and New Year period," he said.
"People should feel confident they can go out and enjoy themselves safely over the Christmas period, albeit dependant on any changes the government may or may not make over the coming days in relation to health protection regulations, or advice regarding the latest variant of Coronavirus."
Police have appealed for any witnesses to the assault on the police officer to get in touch.
They would also like to hear from anyone who was in the area between 9-10pm on Saturday or anyone with dashcam or other footage which may be relevant to get in touch.
Anyone who witnessed the incident or who has information which may assist the investigation is asked to contact police on 101 quoting CR/110192/21.