At least nine people were killed in Nigeria by security forces as protesters clashed with police during mass demonstrations over the country's economic crisis, a rights group said Friday, while authorities said a police officer was killed and several others injured.

Authorities said another four protesters were killed and 34 injured by a bomb in the northeastern state of Borno, where the world's longest war on militancy has left millions displaced and hungry.

More than 300 protesters were arrested and curfews were imposed in four northern states after the looting of government and public properties, Nigerian police said.

National police chief Kayode Egbetokun said Thursday night that the police are on red alert and may seek the help of the military.

Protests over food shortages, corruption

Amnesty International's Nigeria director Isa Sanusi said the group independently verified deaths that were reported by witnesses, families of the victims, and lawyers.

The protests were mainly over food shortages and accusations of misgovernment and corruption in Africa's most populous country. Nigeria's public officials are among the best paid in Africa, a stark contrast in a country that has some of the world's poorest and hungriest people, despite being one of the continent's top oil producers.

Carrying placards, bells and Nigeria's green-and-white flag, the mostly young protesters chanted songs as they listed their demands, including the reinstatement of gas and electricity subsidies that were cancelled as part of an economic reform effort.

WATCH | Police fire tear gas as protesters in Nigeria protest soaring inflation:

Police fire tear gas as protesters in Nigeria protest soaring inflation

Police in Nigeria’s capital, Abuja, turned to tear gas on Thursday as they faced crowds protesting economic reforms and rampant inflation.

Violence and looting were concentrated in Nigeria's northern states, which are among the hardest hit by hunger and insecurity. Dozens of protesters were seen running with looted goods, including furniture and gallons of cooking oil.

Egbetokun, the police chief, said officers "aimed at ensuring peaceful conduct." But, he added, "regrettably, events in some major cities today showed that what was being instigated was mass uprising and looting, not protest."

The police chief's claim was disputed by rights groups and activists.

"Our findings so far show that security personnel at the locations where lives were lost deliberately used tactics designed to kill," Sanusi said.

Authorities feared the protests, which have been gathering momentum on social media, could be a replay of the deadly 2020 demonstrations against police brutality in this West African nation, or as a wave of violence similar to last month's chaotic tax hike protests in Kenya.

However, the threats that emerged as the protests turned violent in some places did "not require that level of response" from police officers, said Anietie Ewang, a Nigerian researcher with Human Rights Watch.