The police do not always need a warrant before they arrest somebody. Unless there is a state law limiting the power of police more than the U.S. constitution would require, it's OK for cops to arrest...
The police must have probable cause, that is, base upon the known information it is likely that either a crime has been or is about to be committed. It is more than a hunch or mere suspicion, but...
(Answer is applicable to the USA only):
A warrantless arrest is never served. A warrantless arrest is performed. Following the warrantless arrest, the arresting individual (typically) or authority...