Advertise them
It's as simple as that. If most of people's contact with the police is irrelevant to actual police job, their opinion will be based on irrelevant factors. So, dress your officers in cool uniforms, let them parade through the cities with their orchestras, show them in TV, on billboards and posters as helpful and smiling. You don't need to invent anything, such posters existed and exist today.
Some examples
USSR poster promoting militia/police. The caption says "Militia - People's servant".
Polish poster promoting militia/police (a while ago). Caption says "Policeman - your friend and defender"
New Zealand's police recruiting campaign that went viral recently.
And yes, that stuff actually works.
Fighting with opposition may sell better then work againsagainst regular crime!
Dystopian dictatorship should have well working-working propaganda. Show opposition as trouble-makers. Present them as working for enemy force. Present statistics of how their actions hurt economy (you don't need to measure anything; just make them up). HighlighHighlight the police role in keeping order.
Extra points: show any opposition actions as hitting people in everyday life, like affecting the price of sugar, or anything in that vein. YouYour goal is no less than to make people blame opposition for everything bad, especially things like hurting teeth and bad weather.
You don't need to invent much, just study a bit of history. Or even look at some propaganda today.