BNP's Assistant Organising Secretary Abdus Salam Azad was assaulted by a group of youths inside the party's Naya Paltan central office in the capital on Sunday.
He was later admitted to Islami Bank Hospital with head injury.
Officials at the BNP central office said a group of youths entered the Juba Dal office housed on the third floor of the central office building Sunday noon and assaulted Salam, also the Senior Vice-President of Jatiyatabadi Jubo Dal, the youth front of BNP.
The youths could not be identified as they somehow managed to flee the scene, they added.
BNP's Assistant Office Secretary Asadul Karim Shaheen told The Daily Observer, "A group of government-sponsored miscreants attacked Abdus Salam. The miscreants later managed to flee."
When asked, Abdus Salam said, "I was busy making the list of detained leaders and activists of our party in my office. At one stage, a group of miscreants entering my office used abusive language and assaulted me indiscriminately."
Reason behind the attack could not be known immediately. BNP, however, condemned the incident and demanded exemplary punishment for the miscreants.
The party's spokesperson Dr Asaduzzaman Ripon in a statement said, "We strongly condemn and protest the attack and demand exemplary punishment for those responsible."