CPB President Mujahidul Islam Selim and General Secretary Syed Abu Zafar inaugurated the congress at Dhaka's Suhrawardy Udyan on Friday by hoisting the party's flag.
A new executive body will be constituted and new policies chalked out through the four day congress.
A closed-door discussion is scheduled to be held at Mahanagor Natyamancha in Dhaka's Gulistan on October 29.
Leaders and activists crossed the archways after police checking amid tightened security.
A total of 26 representatives from 12 countries are joining the congress this year.
The participating countries include India, Nepal, Pakistan, Sri Lanka, Vietnam, China, United States, Germany, North Korea and Russia.
Songs were rendered by Bangladesh Udichi Shilpi Goshthhi at the programme.
Mujahidul Islam Selim, earlier briefing bdnews24.com about the congress, said, "We believe that four major threats - imperialism, communalism, looting and absence of democracy - have been corroding us for the last four decades.
"With increased corruption and discrimination, the living standards of people have been jeopardised. Our democratic rights have been tarnished," he said.
Selim, once an ally of the ruling party, also remarked that the Awami League had gone for a shift away from the spirit of the War of Liberation. He also noted that the BNP was not being able to perform due to opposition from the Awami League.
He also condemned the BNP through alluding to the party’s "Hawa Bhaban" factor.
"For allying with the Jamaat and following the path of terrorism, people have lost faith in them (BNP)," he said.
The last congress of the CPB was held on Oct 11-13, 2012.
Altogether 683 (including 217 women, 17 indigenous and 108 inspectors) representatives are joining the congress this time.
An international seminar on "Imperialism, new liberalism, and religious fundamentalism" will be held at the National Press Club on Saturday with the participation of foreign delegates.