Normally no person can be arrested without order of the Court or warrant. However, Section 151 of the Code of Criminal Procedure, 1973 empowers a police officer to prevent the commission of any cognizable offence (i.e. non-bailable).
A police officer can arrest any person designing to commit cognizance offence without a warrant if it appears to him that the commission of offence cannot be otherwise prevented. The person so arrested by the police officer cannot be detained in custody for a period exceeding 24 hours.
As per facts given in case, some persons are planning to kidnap and murder Rajan which is cognizance offence and hence police officer can arrest such persons without any warrant or order from the Magistrate.