The arrest of a prominent Biafra activist has sparked protests by hundreds of people in Nigeria's south.
Nnamdi Kanu, who supports the creation of an independent Biafra and is the director of a banned radio station, was arrested last month and is still being held despite a court order to free him, his mainly ethnic Igbo supporters say, according to the BBC.
Activists told the BBC that five demonstrators were killed and several injured after the police opened fire in the city of Port Harcourt.
Police spokesman Ahmad Muhammad denied the reports.
"Measures have been put in place to handle the situation in such a way that public peace is not disrupted and to ensure life and property are protected," he told the AFP news agency.
Mr Kanu is also a leader of the Indigenous People of Biafra (IPOB). Another leader, Uchemna Madu, told the BBC that the group was fighting against the "injustice and inequality" ethnic Igbos faced in Nigeria.
"We believe in Nigeria, we have businesses everywhere in the country but we are getting nothing apart from political and social marginalisation," he said.
"Our lives and properties are not secured, we want to live on our own."
Biafran liberation fighters fought a three-year war in the late 60s, which left over one million people dead.