Many of the dead were trapped on the lower deck of the pilgrim boat

Divers pulled at least two bodies from the bowels of a submerged river boat Monday after the overcrowded vessel carrying Muslim pilgrims sank on Thailand's Chao Phraya river, leaving 18 people dead.

The accident happened on Sunday afternoon near the ancient city of Ayutthaya, when a boat packed with pilgrims returning from a mosque hit a concrete bank in strong currents.

Scores of people lined the bank Monday as divers plunged into the swollen, brown waters for the grim task of searching the partially-submerged pleasure boat for the missing.

Distraught relatives wept as the body of an eight-year-old boy was pulled from the wreckage and laid in a white plastic sheet, an AFP team on site reported.

Rescuers used a white board to keep track of those still missing including six children.

Ayutthaya deputy governor Rewat Prasong later said the confirmed death toll stood at 18 with 12 missing and 14 hospitalised. The dead were not thought to include any foreigners.

The boat's captain is in custody and faces a negligence charge after allowing the double-decker vessel to become overcrowded, police told AFP.

"The boat has permission to carry about 50 people but it was carrying more than 100 people," said Ayutthaya police chief Sudhi Puengpikul.

Disaster struck as the boat returned from a memorial for a Muslim scholar at an Ayutthaya mosque.

Television footage showed the boat stopping abruptly as it appeared to hit the concrete bank, and sinking in seconds.

Thai broadcasters showed desperate scenes of the aftermath of the accident as passengers were pulled from the water while rescuers attempted to resuscitate stricken people.

Many of the dead were trapped on the lower deck of the boat, which ran into trouble agonisingly close to safety.

Officials said its proximity to the bank may have prevented the toll from being even higher.