A Boeing 737 passenger plane carrying 62 people is believed to have crashed into the sea shortly after take-off from Indonesia's capital Jakarta.
The Sriwijaya Air disappeared from radars four minutes into its journey to Pontianak in West Kalimantan province.
It is thought to have dropped more than 3,000m (10,000ft) in less than a minute, according to flight tracking website Flightradar24.com.
Witnesses said they had seen and heard at least one explosion.
Fisherman Solihin, who goes by one name, told the BBC Indonesian service he had witnessed a crash and his captain decided to return to land.
"The plane fell like lightning into the sea and exploded in the water," he said.
"It was pretty close to us, the shards of a kind of plywood almost hit my ship."
A number of residents of an island near where the plane disappeared told the BBC they had found objects they thought were from the plane.
Search and rescue efforts were suspended overnight but were due to resume on Sunday.
The Indonesian navy was reportedly deployed to look for the aircraft on Saturday. Navy official Abdul Rasyid told Reuters news agency it had determined the plane's coordinates and ships had been deployed to the location.
The aircraft is not a 737 Max, the Boeing model that was grounded from March 2019 until last December following two deadly crashes.
Last contact with the plane, with the call sign SJY182, was made at 14:40 local time (07:40 GMT), according to the transport ministry. The usual flight time to Pontianak, in the west of the island of Borneo, is 90 minutes.
It did not send a distress signal, according to the head of national search and rescue agency Air Marshal Bagus Puruhito.
There were thought to be 50 passengers - including seven children and three babies - and 12 crew on board, though the plane has a capacity of 130. Everyone on board was Indonesian, officials say.
Relatives of the passengers have been waiting anxiously at the airport in Pontianak, as well as at Jakarta's Soekarno-Hatta International Airport.
"I have four family members on the flight - my wife and my three children," Yaman Zai told reporters through tears.
"[My wife] sent me a picture of the baby today... How could my heart not be torn into pieces?"
According to registration details, the plane is a 26-year-old Boeing 737-500.
It was in good condition, Sriwijaya Air chief executive Jefferson Irwin Jauwena told reporters. Take-off had been delayed for 30 minutes due to heavy rain, he said.
Sriwijaya Air, founded in 2003, is a local budget airline which flies to Indonesian and other South-East Asian destinations.
The plane went missing about 20km (12 miles) north of the capital Jakarta, not far from where another flight crashed in October 2018.
A total of 189 died when an Indonesian Lion Air flight plunged into the sea about 12 minutes after take-off from the city.
That disaster was blamed on a series of failures in the plane's design, but also faults by the airline and the pilots.
It was one of two crashes that led regulators to pull the Boeing 737 Max from service. The model resumed passenger flights in December after a systems overhaul.
The BBC's Jerome Wirawan in Jakarta says the latest events will bring up difficult questions and emotions in Indonesia, whose airline industry has faced intense scrutiny since the Lion Air crash.
Old aircraft, but many still flying
The aircraft which appears to have been lost off Indonesia was a Boeing 737, but it should not be confused with the much more modern 737 Max - which was grounded following two tragic accidents in 2018 and 2019, including one off the coast of Indonesia.
The Sriwijaya Air flight appears to have been carried out by a Boeing 737-500. This plane formed part of the 737 Classic series, the second generation of 737, which was built during the 1980s and 1990s. The plane, seen as a workhorse aircraft for many years, has a strong safety record.
The Sriwijaya Air 737 was 26 years old. While airlines do tend to prefer to fly newer aircraft - because they have lower running costs - it is not uncommon to find planes of this age still in service.
Indonesia, though, has a relatively poor record on aviation safety. For more than a decade carriers from the region were banned from flying into the EU. Until recently, the perception had been that standards were improving, and in 2018 all Indonesian airlines were removed from the EU's blacklist.
That, however, was before the loss of a Lion Air 737 Max in October 2018. Although the design of the aircraft is widely believed to have been the main cause of the accident, investigators also highlighted issues such as poor maintenance procedures and inadequate flying skills as contributory factors.