The call came during the party's 11th Congress.
CPB President Mujahidul Islam Selim called on the 'comrades' to unite.
"I call on all leftists, communists, unite all and form the Left unity and create the Left democratic alternative," he said, addressing the Congress at Suhrawardy Udyan on Friday.
Remembering the names of the great communist leaders, he said if the followers of these leaders scattered across the country unite, nothing can stop the Left from attaining its goals.
Selim said there cannot be any room for anti-Liberation forces in the country's politics and such an atmosphere must be built up where both the government and the opposition parties are pro-liberation parties.
He claimed that the people are fed up with the bourgeois politics in the country and said the leftists must take the responsibility to end this 'intolerable' politics.
"The leftists, please build a movement today with the other democratic forces. The Awami League is unsuccessful; BNP is more so. Please free Bangladesh from the disgrace of this failure," Selim said.
CPB General Secretary Abu Zafar Ahmed also called for the formation of a left alternative power which can satisfy the aspirations of the poor, the middle classes and the working class in Bangladesh.
Lashing out at the government he said, "The government says that there is a flood of development in the country and so democracy is shut in a box for the moment."
Congress preparation committee chairman and presidium member Haider Akbar Khan Rono could not attend the event in person owing to his ill health.
A written statement was read out on his behalf by Presidium leader Lakshmi Chakraborty.
In the statement, Rono said the ruling party has distanced itself from the ideals that had inspired the liberation movement in 1971.
As many as 683 representatives from across Bangladesh have joined the Congress.
Twenty-six representatives from India, Pakistan, Nepal, Sri Lanka, North Korea, Russia and Vietnam have also joined the Congress.