Atipat Wijan’s band was a regular performer at the beer hall in northern Bangkok, where he said the breakers often tripped. So he did not pay much attention to the faint burning smell near the stage late on Sunday night.

Moments later, footage shows, a huge blaze tore out of the entrance of the bar, as if a giant flamethrower were breathing fire. At some point there was an explosion. Some patrons broke glass windows at the front to escape to safety. Dozens of others fled toward the back of the venue, where many ended up dying. Some exits, officials said later, were blocked or otherwise inaccessible.

“There were 11 of us there and two of my colleagues, including my girlfriend, are gone,” Mr. Atipat said in an interview, his voice shaking. “Everything happened so fast.”

By Monday evening, the Thai authorities said, 28 people had died because of the fire and scores more had been injured, more than two dozen of them critically. They did not release information on the victims or on the cause of death.

It was the latest tragedy at an entertainment venue in Bangkok, the Thai capital and a popular international destination that has a reputation for danger because of generally low safety standards.

Emergency workers estimated that around 200 people were inside the venue, Rong Beer Na Lat Phrao, at the time of the fire. About 1,800 square feet in area, the bar was a popular spot for beer and live music in Lat Phrao, a middle-class neighborhood in northern Bangkok, not far from one of the city’s two airports.

“People were lying on the floor, screaming for help,” said Mr. Atipat, a producer with the band, Totsakan, adding that he had been knocked to the ground by an explosion and had light burn injuries on his neck.

On Monday morning, forensic investigators entered the burned premises with flashlights. Inside, tables and plastic stools were covered in ash. Passers-by outside covered mouths and noses because of the biting smell that wafted out.

Juladit Chayaniyayodhin, a fire protection specialist with the Engineering Institute of Thailand, a nonprofit that operates under the Thai king’s patronage, inspected the 53-year-old premises and was preparing to file a report to the authorities. He said that he was alarmed by the cheap insulation foam that had been used to soundproof the venue. He also noted plenty of plastic decoration that seemed to have been hung from the ceiling.

“Both the foam and the plastic are highly flammable and can cause very toxic fumes,” he said. “They should have not been used inside this structure,” he said.

Mr. Juladit’s findings recalled a fire at a Swiss nightclub recently in which about 40 revelers died during a New Year’s celebration. Four years ago, also in Thailand, 20 people died in a fire at a nightclub in Chonburi Province, southeast of Bangkok. That club was found to have been swathed in flammable material used for soundproofing, and the venue lacked proper licenses.

At Rong Beer, Mr. Juladit said his team had seen three exit signs but no emergency lights that could have guided guests to an exit once the electricity went out and the large room went dark.

On social media, the business apologized and offered condolences. The owner of the bar, whom the authorities have not identified, was injured in the fire and was being treated in an intensive care unit.

“He will be questioned as soon as he is medically cleared,” the Bangkok police chief, Kitrat Phanphet, told reporters, adding that two exits at the back of the bar were raising concerns, with one being partly blocked by lockers. The other appeared to have had a table placed near it, according to Bangkok’s governor, Chadchart Sittipunt.

Many of the bodies were found near the toilets in the exit area, the authorities said.

The one-story venue faces a busy four-lane road and had about 25 windows looking on to the sidewalk. All but one were broken.

“People inside broke them with beer bottles to escape,” said Sonkit Torsoi, a motorbike taxi driver stationed in the area who had helped carry the injured to ambulances, his sleeves and orange taxi vest still black from soot. The flames that blazed through the door were “like from a very big gas stove,” he said.

Mr. Sonkit said that most of the people he saw coming out of the pub were young women. “They were pleading rescuers to go help their friends who were still trapped inside,” he said.

Kantima Pemkittikul, 25, who had come to look for a friend who was at the bar on Sunday and whose phone had been out of service since, said, “It’s painful to see how they must have struggled to get out.”

One of those inside was Jarawee Sermsiri, who went by the nickname Cartoon, a graduate from Bangkok University. When her aunt Kanyapat Chokpaisanpanit called her last night she heard the voice of an emergency worker, who said that he had Ms. Jarawee’s phone and then hung up.

Sick with fear, Ms. Kanyapat drove several hours from her home in northeastern Thailand to Bangkok. “I am praying for good news,” she said, sitting on a bench inside the nearby subway station, her suitcase beside her.

In the afternoon, at the morgue of Bangkok’s police hospital, anxious relatives waiting for news about their loved ones were giving DNA samples to help identify the dead. A volunteer organization was unloading coffins, readying themselves to help with transporting the bodies.

Kaewudon Pungpanee, 24, a migrant from Laos who was working at the bar but who managed to escape, said that he had been told to go to the hospital to collect paperwork and claim a compensation for his younger brother Ponpaset, a waiter who was killed in the fire.

“I don’t want any money,” he said, “I just want to bring my brother home.”

Ephrat Livni, Muktita Suhartono and Hannah Beech contributed reporting.