It is the world’s second biggest Muslim congregation after the Hajj, held every winter.
Around three million people are expected to take part in the final prayers conducted by Indian cleric Maulana Muhammad Saad, organizers say.
The devotees were seen walking down to the already full Ijtema ground.
Thousands gathered in alleys and roads, while many got on to boats on the Turag River to join the Akheri Munajat.
More than 6,000 police personnel have been deployed in and around the Ijtema grounds to ensure security for the devotees, said Gazipur's Superintendent of Police Harun ur Rashid.
Holiday has been declared in educational institutions, offices, and factories around the venue.
Since 2011, Bishwa Ijtema is being held in two phases, to ease the accommodation problem.
The second phase of the Ijtema is scheduled to begin on Jan 20 at the same venue and will end on Jan 22.