Law enforcers arrested four BCL men over the incident. The arrestees are district BCL president Abdus Salam Badhon and three activists Ibrahim, Shishit and Ashik.
Additional Superintendent of Police Sheikh Mostafizur Rahman said BCL activist Imran was assaulting schoolboy Suruj Ali, 15, around 11am while the Victory Day parade was going on at the stadium.
On-duty policemen detained Ibrahim as he refused to listen to their request to stop beating the schoolboy.
Being angered at the detention of their fellow, other BCL men tried to snatch Ibrahim from police which triggered a clash.
At one stage, the BCL activists pushed officer-in-charge of Sadar Police Station Rabiul Islam.
The clash left three policemen, including the OC, injured.
Later, police arrested the four BCL men in this connection.