At least 78 Muslim men suffocated or were crushed to death in southern Thailand on Monday as they were being taken to military barracks in packed army trucks after a demonstration, Thai officials said Tuesday.

The news, coming a day after officials said six people were killed when security forces tried to break up the protest, sparked fears of further violence in the already tense south. Muslims, an aggrieved minority in the largely Buddhist country, outnumber Buddhists in that region.

The death toll of at least 84 was Thailand's second highest for an incident this year. On April 28, an assault by security forces killed at least 112 people, most of them Muslims. This week's incident was the latest sign that Prime Minister Thaksin Shinawatra's campaign to quell a 10-month rebellion in the south is failing. The growing crisis could threaten his reelection.

Monday's protest began when hundreds of residents gathered outside a police station in Tak Bai, a district in Narathiwat province, demanding the release of six men accused of giving weapons to Islamic insurgents. Military officials said the crowd ranged from 1,500 to 2,000 people. Army officials said more than 1,300 were detained.

A Justice Ministry official, Manit Sutaporn, told reporters in Pattani that the 78 detainees likely suffocated from being piled on top of each other in the trucks. "We found no wounds on their bodies," he said.

A forensic expert at the ministry, Pornthip Rojanasunan, said the men could have been gagged to death. "They might have had something stuffed in their mouths or nostrils," she told reporters.

"I'm absolutely shocked," said Ahmad Somboon Bualang, a sociologist and retired university lecturer in Pattani. "For people to die like this during Ramadan, in the Muslim fasting month, is tragic. Suffocating to death is . . . more violent than being shot to death. These people suffered greatly in dying."

Television news Tuesday showed rows of mostly young men sprawled on the ground, roped together with their shirts off and hands tied behind their backs. The scene was apparently filmed before the men were placed in trucks to be taken to an army barracks in Pattani, about 60 miles north of Tak Bai.

Thaksin went to the site of the protest and ordered the military to disperse the crowd, a security analyst said. At least 300 troops with assault weapons surrounded the protesters, firing a water cannon, tear gas and live rounds. The demonstrators threw stones, according to wire service reports.

Thaksin suggested Tuesday that the detainees could have died because they were "weak" from fasting during Ramadan, the Associated Press reported.

[On Wednesday, the prime minister acknowledged security forces had made mistakes in their handling of the riot and voiced regret at the loss of life, according to Reuters.

["I can say that the government resorted to gentle measures and did not use force in suppressing the protesters," Thaksin told parliament. "But mistakes happened during the transfer of the arrested people to trucks."]

The government's handling of the incident has prompted fresh concerns of human rights abuses. Natalie Hill, deputy Asia director for Amnesty International, said the allegations that authorities used excessive force to quell the protest "must be immediately investigated." She said that all deaths related to the incident should be investigated and that anyone suspected of being involved with the deaths should be suspended from duty and brought to justice.

Some analysts said they believed the death toll could rise further and might include extrajudicial killings.

The south has been under martial law since January, when guerrillas raided an army camp, killed four soldiers and took 300 assault rifles. A spate of violence followed, including torchings of schools, assassinations of teachers and policemen and, lately, bombings of hotels.

In April, government troops stormed a mosque, killing more than 30 Muslims in a three-hour shootout after machete-wielding assailants attacked 11 police posts in three Muslim-majority provinces. By day's end, more than 100 were dead.

The government has attributed the violence to Muslim separatists, but no Muslim separatist group has asserted responsibility for any of the attacks. On Tuesday, a Web site for the separatist Pattani United Liberation Organization praised those killed in Narathiwat and vowed that "the enemy's capitol shall be burned to the ground."

But no official is sure who is behind the violence or whether any of the attacks are related. What is clear is that Thaksin's approach, a combination of stepped-up security and outreach to the Muslim community, has not quelled the conflict, analysts said.

The six men whose arrest sparked Monday's protest were community defense volunteers, trained and armed by the government ostensibly to protect villagers against Muslim insurgents. They reported this month that they had been robbed of their shotguns. Police investigators concluded that the men had staged the robbery and arrested them.

The resulting protest was a "spontaneous grass-roots demonstration that displayed the continued estrangement between the community and the government," said Paul Quaglia, director of PSA Asia Co., a Bangkok-based security consulting firm.

"The violence and, in particular, the possibility of extrajudicial killings will further inflame the growing Islamic extremist movement in the area and could raise the level of violence considerably, spreading beyond the south," Quaglia said.

Special correspondent Somporn Panyastianpong in Bangkok contributed to this report.

Soldiers arrested hundreds of protesters Monday after they clashed with police in southern Thailand, a majority-Muslim area.A Thai soldier pastes up pictures of dead protesters. Many Thais fear more violence.