At least 13 people drowned when an overcrowded boat capsized early on Friday on a tributary near the mouth of the Amazon River, police said.
Divers were searching the hold of the sunken vessel for about a dozen people who are still missing after rescuers picked up 46 survivors.
Most of the passengers were asleep in the hold when the boat capsized in strong currents on a curve of a river snaking through Marajó Island en route to the port city of Belém, capital of the northern state of Para.
"We don't know how many people are missing because the captain doesn't know how many were on board," the head of the rescue operation, military police Sargent Orivaldo Santos, told Reuters.
The boat had capacity for 25 passengers, though the captain told police he thought there were more than 60 on board for the 10-hour voyage.
Currents were so strong on the Arari river that the boat sank more than a mile (two km) downstream from where it capsized, Santos said.
The passengers were heading to market in Belém to sell their fruit, açaí and shrimp and pick up supplies.