"Security forces killed five villagers in the town of Zogota near N'zerekore on Friday night," said Labile Michel Sonomou, president of the Guinean branch of Lawyers Without Borders.
A police source speaking on condition of anonymity confirmed there were "at least five dead, killed by soldiers who fired real bullets."
The trouble in the town of Zogota, 1,000 kilometres (620 miles) south-east of Conakry, began on Wednesday when villagers vandalised facilities of Vale, which is developing one of Africa's largest iron-ore mines at Mont Simandou.
"The Brazilian company's facilities were vandalised by the villagers who were protesting the recruitment of workers from other ethnic groups, excluding natives," a local government official told AFP.
The town is home to members of the Guerzes and Tomas ethnic groups.
Sonomou said a government delegation of five ministers, including Foreign Minister Edouard Nyankoye Lamah, had visited the site on Friday. After their departure, security forces arrived at around 11:00 pm (local and GMT) to make arrests.
"It was then that the villagers mobilised and tried to resist the security forces who opened fire, killing five people and injuring dozens of others," the lawyer said.
He said about 30 had been arrested and were being held in a military camp in the town while others "are suffering all sorts of torture." Interior Minister Alhassane Conde said he could not confirm the deaths. "I am aware of the violence in Zogota, but no deaths to my knowledge. I can't confirm the deaths," he told AFP.
Guinea, which is trying to move on after decades of military dictatorships and misrule after its first democratic elections ever were held in 2010, has the world's biggest bauxite reserves and is also rich in diamonds and gold.
Ethnic conflict has played a key role in the west African nation's troubled past and violence between the rival Fulani and Malinke rocked the country in the wake of that election, won by Malinke President Alpha Conde.
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