"Those who keep up with privatisation... policies, who threaten workers' lives to reduce costs... are the culprits of the Soma massacre and they must be held accountable," Turkey's Public Workers Unions Confederation (KESK), which represents 240,000 employees, said on its website.
Desperation and anger were rising as hopes fell for dozens of workers still trapped in the collapsed coal mine in the western town of Soma in the Manisa province.
Thousands of protesters clashed with police in Ankara and Istanbul on Wednesday, accusing the government and mining industry of negligence.
Prime Minister Recep Tayyip Erdogan promised investigations would be launched into the causes of the disaster but rejected claims of government culpability, saying that "such accidents happen".
"We have witnessed one of the biggest work accidents in our recent history," Erdogan said after visiting the mine, where grieving relatives of the victims called for him to resign.
- An electrical fault -
Erdogan said figures remained uncertain but mining operators thought around 90 workers were still trapped following the huge explosion at the mine on Tuesday which was believed to have been set off by an electrical fault.
Reports from rescue workers on the scene suggest the figure could be far higher. Most of the victims died of carbon monoxide poisoning.
The premier also appeared to downplay the seriousness of the accident, comparing it to other mining disasters elsewhere, saying "204 people died in the UK in 1862 and 361 people in 1864".
Hundreds of distraught family and friends gathered near the building where Erdogan gave his press conference were outraged, with some kicking his vehicle.
Public anger also spilled onto the streets.
Police used tear gas and water canon to disperse between 3,000 and 4,000 protesters in Ankara's downtown Kizilay Square, as well as thousands of demonstrators in Istanbul.
Earlier in the day, they also used tear gas against around 800 students marching on the energy ministry, and 50 protesters who threw eggs at the mining research directorate in Istanbul, AFP photographers reported.
The disaster has added to the political pressure on Erdogan, who faced mass protests last summer and a huge corruption scandal involving his family and key allies in recent months.
"If the claims of negligence at the mine prove true, it will have a political price. Such a development would render corruption allegations targeting Erdogan's government more convincing," Professor Ilter Turan of Istanbul's Bilgi University told AFP.
- 'No more hope' -
Turkish Energy Minister Taner Yildiz said early on Thursday that the provisional toll had risen to 282 after more bodies were pulled out of the pit.
Raging fires had been hampering efforts by rescue workers still battling to find survivors and rescuers still hadn't been able to reach two underground shafts.
A miner from a different site who joined the effort, Murat Kurkoglu, told AFP on Wednesday: "We will try to save those who are still stuck one by one, but you know very well that there is no more hope. It's finished for them."
Early reports said 787 workers were underground when the blast occurred. By late Wednesday, "close to 450" workers had been rescued, according to the mine operator, Soma Komur Inc, but accounts from rescue workers cast doubt over the numbers.
Erdem Bakin, a doctor with the Search and Rescue organisation, said the transformer that exploded, triggering the collapse had been found. Those between the transformer and the entrance of the mine -- around 70-80 people -- survived.
"But those who were beyond were taken by the fire and they are all dead," he said.
Harun Unzar, a miner at the site, said: "We are a family and today that family is devastated. We have had very little news and when it does come it's very bad."
- 'Tired of funerals' -
Explosions and cave-ins are common in Turkey, particularly in private mines, where safety regulations are often flouted.
Turkey's previous worst mining accident happened in 1992 when 263 workers were killed in a gas explosion in a mine in Zonguldak.
A lawmaker from the main opposition Republican People's Party (CHP) said it submitted a parliamentary motion 20 days ago to investigate work-related accidents at coal mines in Soma but it was rejected by the government.
The CHP's Manisa deputy Ozgur Ozel told local media: "We receive tip-offs every day that workers' lives are under threat.
"We lawmakers from Manisa are tired of going to miner funerals."
Turkey's ministry of labour and social security said the mine had been inspected eight times in the last four years, most recently on March 17, and was found to comply with safety regulations.
But Oktay Berrin, a miner, said: "There is no security in this mine. The unions are just puppets and our management only cares about money."
Mining company Soma Komur said it had taken maximum measures to ensure safety.
Soma is a key centre for lignite coal mining and is located around 480 kilometres (300 miles) southwest of Istanbul.