Bangladeshis say that the Biswa Ijtema is the largest annual gathering of Muslims outside Saudi Arabia, with two to three million people from Bangladesh and elsewhere expected.
Farmer Abdul Rahman says the festival's popularity stems from its accessibility for the poor.
The Biswa Ijtema, which takes place beside the Turag river, is organised by missionaries and focuses on learning and prayer.
Twelve-year-old religious college pupil Delwar says he has come to his first Biswa Ijtema to get closer to God.
The security forces have deployed 20,000 men, including members of the paramilitary Rapid Action Battalion, to make sure there is no trouble.
Rakib, who has come with his father from London, described the atmosphere as "spiritual".
Getting to the gathering is a struggle for many. Vehicles queued from more than 5km (three miles) away to reach the site, so many decided it was best to walk.
"We have come here to find out how we can best attain peace in this life, and the one after death," Abu Jaffar, from Gazipur, explains. (Pictures and text by Mark Dummett)