Sutopo Purwo Nugroho, a spokesman for the country's disastermanagement agency, said the sprawling concrete bridge on the remote island of Borneo was being repaired when Saturday's accident occurred.
Workers were tightening screws and bolts when a steel support cable snapped and the bridge, which was less than 10 years old, came smashing down.
Four bodies were pulled from the water, including a 6-month-old baby, and 19 people were rushed to the hospital with injuries, said police Capt. Syafii Nafsikin. Scores of people including six of the repair workers were missing.
The 700-meter structure built to resemble the Golden Gate Bridge in San Francisco linked the towns of Tenggarong and Samarinda in East Kalimantan province.
It was clogged with traffic when the accident occurred, Syaiful, a witness, told local television station TVOne.
He saw at least one bus and close to a dozen motorcycles plunge into the Mahakam river. Several cars were mangled.
"Everyone was screaming," said Syaiful, who like many Indonesians uses only one name. "Some survivors were swimming to the shore in panic."
President Susilo Bambang Yudhoyono ordered government ministers to the scene to oversee search-and-rescue operations. He also called for a full investigation into the cause of the collapse.