The Mancha's Spokesperson Imran H Sarker told bdnews24.com that he and some five other members of the secular platform were injured in the attack on Thursday evening.
According to him, around 12 people attacked the demonstrators with sticks and brickbats immediately after the programme ended.
"The attackers alleged that we were lying and there were no floods," he said.
He has started a case against 10 to 12 unidentified people at Shahbagh Police Station.
Sarker and Ganajagaran Mancha activist Sanatan Ullash had turned themselves in to the court on that day on charges of defaming Prime Minister Sheikh Hasina.