ARREST - PHYSICAL RESTRAINT PUT ON A PERSON WITH ALLEGATION (FIR)
DETENTION - PHYSICAL RESTRAINT PUT ON A PERSON WITHOUT ALLEGATION (NO FIR)
Above definition as per State of Punjab v Ajaib Singh, AIR 1953 SC 10.
Hence in my view, a police officer can legally detain a person for a period of less than 24 hours without levelling any allegation on the detainee in view of S.57 of Cr.P.C.
Technically, an arrest is the beginning of the detention. But, In the absence of any proof like FIR ETC. one cannot say a person is arrested in the strict legal sense. Therefore in the absence of proof of arrest, there can be detention which may not be illegal if it is less than 24 hours. However if one can prove detention of more than 24 hours and there is no FIR or proof of arrest the concerned police officer would be liable for criminal consequence.