The arrestees are Mamun, said to be a constable of Bangshal Police Station, and his friend Jitu.
Abul Kalam Azad, the victim, said Mamun and Jitu waylaid him around 3:30pm when he was returning after withdrawing Tk 10 lakh from a bank.
The duo identified themselves as policemen and asked him to ride their motorcycle, he said.
As he refused to abide by their order, they hit him in the head with handcuffs and helmet, leaving him injured.
At one stage, the duo snatched his cash-filled bag and tried to flee.
Noticing the incident, locals came forward, chased the duo and managed to catch them.
Rakibul Hasan, a witness, said though they claimed themselves as policemen, they failed to show any identity card.
Ershad Hossain, a sub-inspector of Motijheel Police Station, said police took them to their custody.
They are looking into what happened there actually, he said.
Meanwhile, the victim was admitted to Dhaka Medical College Hospital.