Lagos - Tensions mounted between Nigerian security forces and pro-Biafran supporters on Thursday, after renewed protests calling for independence and clashes targeting the police.In the capital of the southern state of Rivers, Port Harcourt, 32 people were arrested after two days of demonstrations and the death of one police officer.In neighbouring Abia state, supporters of the Indigenous People of Biafra (IPOB) group targeted police in the commercial hub, Aba, and the capital, Umuahia.Police vehicles were seen with smashed windscreens and officers fired warning shots, while there were reports of petrol bombs thrown and bonfires set to restrict movement.A police station in Aba was burnt down between 06:00 am and 7:00 am on Thursday, said state police spokesperson Geoffrey Ogbonna."We are investigating the incident but no arrest has been made," he told AFP but denied reports that the commissioner of police's residence in Umuahia was attacked.A three-day dusk-to-dawn curfew was imposed in Abia on Tuesday to prevent clashes after IPOB leaders said troops killed five of its members.The army has denied the claims.Abia state governor Okezie Ikpeazu said he recognised the presence of troops had been a cause of "great concern" but they would be withdrawn from Aba and Umuahia by Friday."With the expected exit of soldiers from the streets, we must warn that we will not tolerate agitators and protesters taking over the streets for any reason," he added. Abuse claims IPOB leader Nnamdi Kanu has been charged with treasonable felony and is currently on bail pending the resumption of his trial in the capital, Abuja, next month.His arrest in October 2015 and detention was the catalyst for a new wave of demonstrations calling for independence for the Igbo ethnic group that dominates the southeast.The issue is a sensitive one in Nigeria, as a previous unilateral declaration of an independent republic of Biafra in 1967 sparked a bloody civil war that lasted 30 months.More than one million people died, most of them Igbos, from the effects of war, famine and disease.Kanu, who also runs Radio Biafra, has vowed a non-violent approach to force a referendum on self-determination."We have chosen the track of peaceful agitation, non-violence, persuasion, logic, reason, argument," he told AFP in an interview in May.But an enhanced military presence in the region since the start of this week has stoked deep-seated tensions.Officially, the military says it has deployed troops as part of an operation - codenamed "Python Dance II" - to tackle rising crime.A video clip has been circulating online, purportedly showing soldiers punishing IPOB members by forcing them to drink muddy water while stripped to the waist.The army said it would look into the claims, vowing that any soldier found guilty of breaching its code of conduct "will face (the) full wrath of the military justice system".In Port Harcourt, Rivers state police spokesperson Nnamdi Omoni said nine IPOB supporters were arrested after a sergeant attached to the force riot squad was killed on Wednesday.IPOB members allegedly seized his rifle. Several other officers were injured and a patrol van was burnt in the incident.Twenty-three others were held on Tuesday after an attack on the city's police training school, he added.Nigeria has long suffered sporadic flare-ups of ethnic tensions but President Muhammadu Buhari has said the country's unity is non-negotiable.Human rights groups have accused the military of abuses in trying to maintain order, claiming that at least 150 pro-Biafra supporters were killed in the last two years.Hundreds more have been injured.