Do the public have the power to arrest under the law?

The answer to that question is Yes. Citizen's arrest (tangkapan oleh orang awam/orang persendirian) is legal under the law. The relevant provisions for that answer are stipulated in the Criminal Procedure Code (CPC).

According to Section 27 (1) of the CPC, any private person may arrest any person who in his view commits a criminal offence and shall without unnecessary delay hand over the person arrested to the nearest police officer or to the nearest police station. 

Based on Section 27 (2) of the CPC, after the private person handed over the arrested person to the police officer and there is good reason to believe that the arrested person had committed an offence then the police officer shall rearrest him (membuat tangkapan semula).

However as in Section 27 (4) of the CPC, if the police officer has a reason to believe that the arrested  person has not committed any offence, then the arrested person shall be released at once.

Question remains whether the public is entitled to cause injury or harm to the person arrested? The answer is definitely No. You may be charged for causing hurt under Section 323 of the Penal Code. You can only use reasonable force that is necessary to detain the person.