Government has promised to fully investigate after 37 miners died while being held in police custody in Niger state.

Security forces in Nigeria have opened fire and deployed tear gas during protests over the deaths of 37 miners in custody.

Demonstrators rallied in their hundreds in the mineral-rich city of Minna in Niger State during a second day of protests on Friday.

It was not immediately clear whether troops fired live ammunition or anti-riot pellets, but a witness told the Reuters news agency that bullets had been used and the body of one person lay on the ground.

A 24-hour curfew has been imposed by state governor Umar Bago, while the government has promised to fully investigate the 37 deaths.

Officials announced on Thursday that they died while in the custody of the Nigeria Security and Civil Defence Corps (NSCDC), a paramilitary agency.

The miners were arrested earlier this week during raids in the Wushishi-Lukoto area, west of Minna, on suspicion of illegal mining.

The NSCDC initially suggested that the miners may have been killed by an outbreak of disease, but has since clarified that the cause of death is yet to be confirmed. The NSCDC said further investigations will be conducted, including medical examinations.

However, all of the deceased were buried on Thursday evening, raising questions about whether post-mortems will still be possible.

The AFP news agency said it had seen an intelligence report suggesting that the deaths may have been caused by poor ventilation in the cells and overcrowding. Some protesters also told the Associated Press that the miners had suffocated.

Some survivors recounted their panic on Friday, saying they struggled to breathe inside the cell.

“We were jam-packed in the cell, no ventilation for us to breathe well,” Dauda Shehu told journalists at a central Nigerian hospital. “All of a sudden we realised that we could not breathe and we were all scampering for where to get fresh air, we even appealed to them several times, banged the cell door, but they refused to answer us.”

The NSCDC’s commandant in Niger State, Suberu Siyaka Aniviye, has been suspended by Interior Minister Olubunmi Tunji-Ojo, pending a full investigation.

“It is an unfortunate incident, however, a full investigation will be conducted while the commandant under whose watch this happened remains suspended,” the Interior Ministry said in a statement.

Niger State is home to vast deposits of mineral resources, including gold and uranium and mining is a core part of the economy.

Illegal mining is common in Nigeria and has been blamed for fuelling violence among armed groups competing for control of the lucrative trade.