Several people are feared dead in Nigeria – with dozens more injured – after a protest over the continued detention of Biafra leader Nnamdi Kanu turned bloody. Soldiers and police were accused of firing on unarmed protesters as they gathered at a school in the city of Aba yesterday (9 February).

The number of reported deaths is unconfirmed, with figures reaching as high as 16 fatalities. Biafra supporters say about 30 others are receiving treatment in hospital, while another 20 had been detained by police.

The unrest came as thousands were due to take part in a pro-Biafra "mega protest" across south east Nigeria. The three days of planned action – between 9 and 12 February – coincides with the trial of pro-Biafra figurehead Kanu, who has been accused of treasonable felony charges.

As protesters gathered at the National High School in Aba, eyewitnesses reported seeing state security vans arrive and armed officials scale the perimeter fence to break up the demonstration. Biafra supporters say soldiers then shot at unarmed protesters, causing panic and a stampede. A defence spokesman for Nigeria denied the allegations, saying nobody had been killed.

"The protesters as usual, gathered at the school to pray before taking off, but soldiers and policemen invade the area," an eyewitness told Nigeria's Vanguard. "There was heavy shooting. I saw more than three dead people. Many others were injured."

Another protester added: "This is what the Nigerian army are doing to us. We gathered this morning for a peaceful demonstration. This whole place is now on fire."

Video footage published online by Biafra supporters showed the aftermath of the incident, with the school's playing field set alight by tear gas canisters and the ground stained by what was said to be the blood of those allegedly shot.

An umbrella group of NGOs – the Abia Human Rights Agenda – said at least 10 protesters had died, with the group calling for the army and police to launch an investigation into the incident. In a statement published by Vanguard, it said: "Security agents must restrict themselves to the rules of their engagements in crisis situations."

The incident came the same day Kanu, the UK-based director of Radio Biafra and leader of the Indigenous People of Biafra (Ipob), arrived in handcuffs at the Federal High Court in Abuja, Nigeria. He was arrested by the State Security Service (DSS) in Lagos in October 2015 and faces charges of treasonable felony – allegations he denies.

Kanu has become a figurehead for the pro-Biafra movement, which seeks the independence of the Biafran territories forcibly annexed to Nigeria during the British colonisation, which ended in 1960. A Biafran Republic was established in 1967 and reannexed to Nigeria in 1970, following a civil war that claimed between one and three million lives.

Update 12 February:

Speaking to IBTimes UK, defence spokesperson Rabe Abubakar denied the police and army killed demonstrators. However, he also said security forces had to intervene as some pro-Biafrans were allegedly carrying weapons.

"The issue of security forces shooting some people should not come up because I don't know where you found that they were harmless," he said. "To my own definition, someone who is carrying dangerous weapons, including guns, is dangerous and is not allowed to do so.

"Yes, they have the right to protest, but the Nigerian constitution prohibits people or groups from threatening the stability of the country. We are here to protect people anywhere in the land and we cannot in any way authorise escalation of violence.

"We have one Nigeria and nobody can undermine its unity, it is a constitutional prohibition to call for a separation and it goes against the continuity of our country."