Lagos (AFP) - At least 10 people were killed during protests by pro-Biafra campaigners as they marked the anniversary of the start of Nigeria's civil war, police said on Tuesday.

Activists wanting a separate state for the Igbo people in the southeast were commemorating the 49th anniversary of the declaration of an independent Republic of Biafra on May 30, 1967.

The civil war that followed lasted until January 1970 and left more than one million people dead, many of them from starvation and disease.

Police said officers opened fire because members of the Indigenous People of Biafra (IPOB) movement shot at security forces deployed to monitor the protests.

But IPOB spokesman Anayo Chukwu-Okpara denied the claim and said at least 35 members of the group were killed in the commercial hub of Onitsha in Anambra state.

An Anambra police spokesman told AFP "five corpses were recovered" in Onitsha while in the capital of neighbouring Delta state, Asaba, the police said five protesters were killed.

"We had to deploy our officers to ensure that the protest was peaceful but we were surprised that the people turned violent," said Delta police spokesman Charles Muka.

There was also violence in the capitals of Imo, Ebonyi, Abia and Rivers state, he added.

- IPOB claims -

IPOB leader Nnamdi Kanu has been in custody since his arrest last October on charges of treasonable felony, which has sparked protests demanding his release.

The government has repeatedly refused to release him despite several court rulings granting him bail.

Kanu's brother, Prince Emmanuel Kanu, said of the Onitsha protest: "We are not armed, we only came to remember people who died between 1967 to 1970, that was just our crime to open fire on us.

"What happened on Sunday night at the ground we were supposed to use for the remembrance day, on that evening they opened fire, they shot about 20," he said.

He said on Monday during the march, 16 people were killed and 56 people were injured.

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"They blocked everywhere and all of a sudden they started shooting at us, they shot 56 people, who are injured."

- Police crackdown -

Federal police chief Solomon Arase on Tuesday said any IPOB member found with a firearm would be arrested and tried, while those held in connection with police deaths would be charged with murder.

The police "will continue to diligently work towards eliminating any threat to internal security", he added.

Since Kanu's arrest, positions have hardened on both sides. President Muhammadu Buhari, a former military ruler, has publicly stated he "will not tolerate" any threat to Nigeria's unity.

IPOB members told AFP recently that armed police and soldiers released tear gas and shot "indiscriminately" at crowds with live ammunition at previous protests.

There have also been allegations the dead were buried in mass graves and of disappearances.

Similar claims have been made against the military in its response to unrest involving Shiite Muslim protesters in the northern city of Zaria last year and in the Boko Haram conflict in the northeast.

Human Rights Watch described the latest clashes as "a wake-up call to urgently de-escalate the situation" and called for a "credible judicial investigation" to determine what happened.

"In this context of escalating tensions in southeast Nigeria, government security forces should ensure they take reasonable steps to protect security and avoid excessive use of force in controlling public demonstrations," said HRW's Mausi Segun.

"Leaders of the protest groups should redouble their efforts to inform demonstrators to act peacefully and to condemn those who do not," she added in an emailed statement