A police officer can legally arrest you without a warrant in Canada, in some instances. However, a warrant must usually be issued by the court in order for the police to arrest you.
A police officer in Canada cannot simply stop and search you without reasonable grounds to believe that you have been involved in a crime. The police must follow the correct procedure and respect your rights under the Canadian Charter of Rights and Freedoms. Even if the police do have the right to search you, there are guidelines for how this search should be conducted.