At least 78 people died in southern Thailand after being arrested and loaded into army trucks following Monday's clashes with security forces.
Protesters were arrested and many then loaded into army trucks
Officials said almost all the dead suffocated as they were taken to an army barracks several hours away.
The new death toll is in addition to six people who died during the clashes themselves, which erupted on Monday following the arrest of six Muslim men.
It was the latest in an upsurge of violence in the Muslim-majority south.
Violence erupted after at least 1,500 protesters gathered outside a police station in Narathiwat province after the detention of six Muslim men.
After clashing with security forces, more than 1,000 protesters are reported to have been arrested.
0900 Monday (0200 GMT): 1,000 protesters gather at Takbai police station
Army urges protesters to disperse, but instead hundreds more join
1500: Protesters hurl objects at police, who use tear gas and fire bullets
Hundreds arrested and six deaths reported
1800 Tuesday: Officials admit 78 others died in army trucks after arrest, many of suffocation
Thai officials said 78 of the arrested protesters were later found dead, apparently after they suffocated in trucks used to transport them.
"After we brought people who were arrested into detention, we found that another 78 people were dead," Justice Ministry spokesman Manit Suthaporn told reporters.
Dr Pornthip Rojanasunan, a forensic expert for the Justice Ministry, told the BBC that 80% of the victims died from smothering or suffocation and 20% from stress or convulsions.
Army deputy commander Maj-Gen Sinchai Nujsathit admitted that "we had more than 1,300 people packed into the six-wheel trucks" for a journey to Pattani province that took five hours.
Mr Suthaporn said the men were already weakened from fasting and that when they were piled on top of each other they probably could not breathe.
More violence feared
The violence began on Monday after a crowd gathered at the district police station in Takbai, Narathiwat, to protest against the detention of six men accused of providing weapons to Islamic militants.
Police said they fired water cannon and tear gas to disperse the crowd. Some reports also said police used live ammunition.
Southern police chief Manote Graiwong told Bangkok radio earlier that protesters were being questioned to find out who persuaded them to gather and how they were mobilised.
He said that many of the demonstrators were armed and appeared to have taken drugs.
A local Islamic leader said he was in shock after being informed of the latest death toll.
"I cannot say what [is] going to happen, but I believe that hell will break out," Abdulraman Abdulsamad, the chairman of the Islamic Council of Narathiwat, told The Associated
But Thai Prime Minister Thaksin Shinawatra, who flew to the south after the clashes, has praised the security force's response.
"They have done a great job," he told reporters, before the higher death toll was announced. "They (the protesters) really set out to cause trouble so we had to take drastic action against them," he said.
Neighbouring Malaysia, on the border with southern Thailand, expressed concern.
"We hope the government of Thailand will be able to manage this crisis so that it will not spread and inflame further violence," Prime Minister Abdullah Ahmad
Badawi told reporters.
More than 400 people have died this year in clashes between militants and security forces in Thailand's Muslim-majority southern provinces.
Muslims in the south have long complained of discrimination, and civil servants and security officers have been targeted in a wave of violence which began in January this year.
Muslim separatists fought a low-key insurgency in the region in the 1970s and 80s.