LONDON and LAGOS -- Amnesty International said it has received "credible but disturbing evidence" of security forces killing protesters who were demonstrating against police brutality in Nigeria's largest city.
"While we continue to investigate the killings, Amnesty International wishes to remind the authorities that under international law, security forces may only resort to the use of lethal force when strictly unavoidable to protect against imminent threat of death or serious injury," the London-based human rights organization wrote on Twitter late Tuesday.
Lagos, the sprawling financial hub of Africa's most populous country, has been the center of weeks-long, nationwide protests over a now-disbanded, widely-criticized police unit, the Special Anti-Robbery Squad (SARS), which has been accused of human rights abuses. The demonstrations have been largely peaceful, but tensions have spiraled in recent days and authorities have imposed an indefinite 24-hour curfew in Lagos and other parts of Nigeria.
The Lagos state commissioner for information, Gbenga Omotoso, said Tuesday that "there have been reports of shooting" at the Lekki toll gate, one of the main roads into Lagos's business district, following the announcement of the curfew. Hundreds of protesters have been gathering at the toll gate in Lekki, a wealthy suburb of Lagos.
"The State Government has ordered an investigation into the incident," Omotoso said in a statement posted on his Twitter account. "Governor Babajide Sanwo-Olu has advised the security agents not to arrest anyone on account of the curfew, which he urges residents to observe for the peaceful atmosphere we all cherish."
Video shown on Nigeria’s Channels Television appeared to capture the sound of live rounds being fired at the scene.
Leaders of the protest movement, which uses the social media hashtag #EndSARS, claim the Nigerian government ordered the removal of surveillance cameras at the Lekki toll gate and for the lights to be shut off before directing security forces to open fire on protesters there Tuesday night.
The Nigerian Army has denied that any of its personnel were involved in the reported incident.
Lagos State Gov. Babajide Sanwo-Olu has condemned the alleged shooting, saying in a statement Wednesday that "there are no excuses." He has also warned that the growing protests have "degenerated into a monster that is threatening the well-being of our society."
“Lives and limbs have been lost as criminals and miscreants are now hiding under the umbrella of these protests to unleash mayhem on our state,” Sanwo-Olu said in another statement posted on his official Twitter account Tuesday.
The governor said one person who was recently admitted to a Lagos hospital has died "due to blunt force trauma to the head."
"This is an isolated case. We are still investigating if he was a protester," Sanwo-Olu tweeted Wednesday.
There were more than a dozen others who remained hospitalized "with mild to moderate levels of injuries," he tweeted earlier.
The Lagos state government has ordered the indefinite closure of all public and private schools amid the unrest. Meanwhile, the U.S. Consulate General in Lagos remained shut Wednesday after closing its doors a day earlier due to the violence.
"Although most demonstrations are peaceful, some have become violent and have shut down major thoroughfares and bridges," the consulate said in a statement Tuesday. "We continue to urge all U.S. citizens to avoid areas around protests and demonstrations and to check local media for updates and traffic advisories."
Armed crowds attacked two correctional facilities in Edo state on Monday, freeing nearly 2,000 inmates, according to a statement from Mohammed Manga, spokesman for the Nigerian Ministry of Interior, which said the perpetrators were "protesters purportedly under the #EndSARS aegis." There have also been attacks on police stations in Lagos state, according to the governor.
Gunshots were heard again in Lagos on Wednesday as some protesters continued to demonstrate despite the curfew. People set fire to a television news station and part of the Nigerian Ports Authority headquarters.
Nigeria's Inspector-General of Police Mohammed Adamu has ordered the nationwide deployment of anti-riot police and has advised the Nigerian Police Force to "exercise the full powers of the law to prevent any further attempt on lives and property of citizens," according to a statement.
Beyonce, John Boyega, Naomi Campbell and Rihanna are among the celebrities who have spoken out in support of Nigeria's #EndSARS movement and have called for an end to the violence. Rihanna posted a photo on her Instagram account, showing a protester holding up a blood-soaked Nigerian flag.
Former U.S. Vice President Joe Biden, who is the Democratic presidential nominee, issued a statement late Tuesday urging Nigeria's president and military "to cease the violent crackdown on protesters."
"The United States must stand with Nigerians who are peacefully demonstrating for police reform and seeking an end to corruption in their democracy," Biden said. "I encourage the government to engage in a good-faith dialogue with civil society to address these long-standing grievances and work together for a more just and inclusive Nigeria."
Last week, as protesters showed no signs of backing down, Nigerian President Muhammadu Buhari stepped in and dissolved SARS, which operated across the country -- often in plainclothes -- and has been accused of assault, extortion, extrajudicial killings, kidnapping, torture, unlawful detentions and robbery.
"The disbanding of SARS is only the first step in our commitment to extensive police reform in order to ensure that the primary duty of the police and other law enforcement agencies remains the protection of lives and livelihood of our people," Buhari, who is a retired general of the Nigerian Army, said in a statement on Oct. 12. "Meanwhile, it is important to recognize that the vast majority of men and women of the police force are hardworking and diligent in performing their duties. The few bad eggs should not be allowed to tarnish the image and reputation of the force."
ABC News' Conor Finnegan and Joseph Simonetti contributed to this report.