The Universal Declaration of Human Rights is a document created by the U.N. in 1948, after World War II, and is considered binding by international law in most U.N. member-countries. Article 9 states:
No one shall be subjected to arbitrary arrest, detention or exile. (un.org)
Assuming the U.N. or a similar body still exists in the world of Fahrenheit 451, this Article has been eliminated entirely; people are routinely arrested, exiled, and even executed without trial or any due process. Montag discovers the truth of this at the end of the book, when he is forced to destroy his own home and books, and threatened first with arrest and then with execution. Earlier, Clarisse mentions an incident in her family's life:
"What's going on?" Montag had rarely seen that many house lights.
"Oh, just my mother and father and uncle sitting around, talking. It's like being a pedestrian, only rarer. My uncle was arrested another time -- did I tell you? -- for being a pedestrian."
(Bradbury, Fahrenheit 451, Google Books)
Since cars are automated and extremely fast, people don't often walk except between subways and short distances. Pedestrians are so rare that the government sees them as subversive; arresting someone for no reason other than not being in a car is certainly a violation of Article 9. However, because of the control wielded by the government, it is likely that any proper declarations of human rights have been eliminated along with record of their existence, and so this sort of arrest is seen as normal.