Local television footage showed people standing on an overturned boat trying to reach a fishing boat packed with survivors.
The Evelyn Calista 01 was carrying 72 passengers, mostly people returning from visits to their hometowns to celebrate Eid al-Fitr holiday with families, and six crew members, Sidhakarya said.
The boat sank Thursday afternoon about three hours after leaving a port in Tembilahan, a town in the Indragiri Hilir Regency in Riau province. It was bound for Tanjung Pinang city in a neighboring province in the Riau Islands chain, a 200-kilometer (124-mile) trip.
The cause of the sinking is still being investigated, but some survivors told authorities the boat swayed suddenly and capsized after hitting a large log while sailing in strong winds, local police chief Norhayat said.
Two tugboats and two inflatable boats have been searching for the missing people since Thursday night, battling high waves in the overnight darkness, said Norhayat, who goes by a single name, adding that a tugboat pulled the capsized boat to a port before dark Thursday to ease the search efforts.
Boat tragedies are common in Indonesia, an archipelago of more than 17,000 islands, where ferries are often used as transport and safety regulations can lapse.
In 2018, an overcrowded ferry with about 200 people on board sank in a deep volcanic crater lake in North Sumatra province, killing 167 people.
In one of the country’s worst recorded disasters, an overcrowded passenger ship sank in February 1999 with 332 people aboard. There were only 20 survivors.