Arizona has just passed a new Immigration law that will allow police to demand proof of citizenship or legal residence without any just cause other than they suspect the person is an illegal immigrant. The law demands that everyone (or in reality those who look Hispanic) must carry proof of legality at all times or be subject to arrest.
I grew up in South Africa under Apartheid where blacks were forced to carry a "pass" which authorized them to live in certain areas. The police would randomly stop blacks and if they didn't possess a pass or even if they did not have it on their person they would be arrested and shipped back to a remote "homeland". If all the travails imposed by Apartheid on the black population that made up 80% of South Africa, this law was the most reviled. In 1960 the police shot and killed 69 unarmed peaceful protesters because they dared to burn their passes in a protest.
The law in Arizona is essentially no different. In the Phoenix area where Crazy Sheriff Joe Arpaio, currently under federal investigation for discriminating "search and seizure" rules and for corruption will no doubt have his police arbitrarily pulling over anyone they suspect might be an "illegal" which means focusing only on Hispanic looking people. To make matters worse, anyone can sue the police department if they think they are not doing enough to go after illegal immigrants. Anyone in the company of an 'illegal' is also subject to arrest whether they are aware of it or not.
The law is in all likelihood unconstitutional and I am sure it will end up in the courts but with the current Supreme Court there is no guarantee that it will be struck down.
More details here. The implications are far worse than even my short description suggests.