Do the police need an ‘arrest warrant’ to arrest me?

No, they do not need a warrant if they suspect that you committed a serious crime (cognizable offence). Examples include murder, sexual offences, acid attack, rioting, starting a fire etc. Normally, it is the Magistrate who will issue a warrant to arrest you. Since these type of crimes need urgent action from the police, the police can take charge even without permission from the Magistrate.