Working in the police is like working in any other job; some of the things involved are permissible and others are haraam - that varies according to the nature of the work that is done by the person who has that job.
In many countries, unfortunately, those who work in this field are required to shave off their beards, salute the flag, and show respect to officers of higher rank by greeting them in a manner that is not Islamic, even if they are disbelievers or evildoers. We have explained why these things are haraam in the answers to questions no. 5481, 8230 and 8797.
With regard to that work, it is well known that it involves some kinds of wrongdoing as well as some kinds of justice. If a person's work involves pursuing criminals and evildoers, protecting people from their harm, returning to people what is rightfully theirs, and preventing wrongdoers from transgressing against others, then undoubtedly this work is permissible and in fact he will be rewarded if he makes his intentions sincerely for the sake of Allah, may He be exalted, by aiming to establish justice and prevent wrongdoing. But if a person's work involves taking people's wealth away from them and transgressing against them by beating them, impugning their honour and imprisoning them unlawfully, then undoubtedly his work - in this case - is haraam, and he is one of the bankrupt who will meet Allah, may He be exalted, in the Hereafter with people having claims against them, so Allah, may He be exalted, will give them hasanaat (good deeds) that He will take from those wrongdoers, or some sayi'aat will be taken from those who were wronged and added to the burden of (those who wronged them).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
If the ruler takes from the collectors of revenue that which he and his family want to keep for themselves, it is not right to help any of them, because each of them will be a wrongdoer, like a thief who steals from a thief and like two groups who fight one another on the basis of factionalism and for positions of leadership. It is not permissible for anyone to help in wrongdoing or transgression. Helping one another is of two types:
1. Helping one another in righteousness and piety, such as jihad, carrying out hadd punishments, giving people their rights in full and giving to those who are deserving. These are things that are enjoined by Allah and His Messenger (blessings and peace of Allah be upon him). The one who refrains from that for fear of helping in wrongdoing is neglecting a duty that is either an individual or communal obligation, thinking that he is being cautious. But how often cowardice and failure are presented caution because in each case one ends up refraining from taking action.
2. Helping one another in sin and transgression, such as helping to shed the blood of one who is protected (by shareeah), or property that is protected (by shareeah), or beating someone who does not deserve to be beaten, and so on. This is what is forbidden by Allah and His Messenger. End quote.
Majmoo al-Fataawa, 28/283
See also the answer to question no. 103412
It may be said concerning such jobs that if the one who takes on such a job has done so in order to call people to what is good and to reduce evil, and to alleviate wrongdoing suffered by those who are wronged, and to benefit people as much as he can, then in that case there is nothing wrong with this job.
But if the one who takes on this job has done it in order to take advantage of his position to mistreat people, acquire money unlawfully using his post as a shield (against prosecution), accept and defend man-made laws, defend wrongdoers, and to transgress against the innocent, and other kinds of wrongdoing that may be committed by those who do this work, then undoubtedly it is haraam work, and there is the fear that the one who does that may come on the Day of Resurrection bankrupt, with no hasanaat (good deeds) to his credit, and carrying a mountain of sins.
We ask Allah to keep us safe and sound.
And Allah knows best.
Reproduced from Islam QA