You never know what could happen when you have two opposing groups. I’m sure the Christians were outnumbered and they very well could have been attacked . The job of the police is to make sure that the two opposing groups do not intermingle, thus preventing violence . It appears that the Christians did not obey the law and were arrested for it .
Keeping the peace is the goal of law enforcement whenever there is a politically oriented parade/demonstration . No matter how unpopular, or whether they agree with it or not, the police have a specific duty. I seriously doubt persecuting Christians is what the police were doing, as that vague article would have you believe .