DAKAR, Senegal — Global outrage mounted Wednesday after security forces in Africa's largest city opened fire into a crowd of protesters, deepening unrest spurred by anger at Nigerian police.

Ten people died and dozens were wounded after uniformed men took aim at demonstrators the night before at a Lagos toll gate plaza, Amnesty International said, a clash captured from multiple angles on social media.

The violence followed two weeks of largely peaceful demonstrations that prompted Nigerian President Muhammadu Buhari to dissolve the undercover police unit at the center of the dispute and that critics have long blasted as abusive.

But hundreds returned to the streets Wednesday — despite a 24-hour curfew enforced by riot officers — and thousands more joined solidarity marches in other countries, saying past attempts at ending police brutality in Nigeria had fallen short. Protesters in Lagos, a metropolis of approximately 20 million, said they would not stop until wrongdoers in law enforcement are brought to justice.

Politicians and celebrities around the globe sent words of support to the protesters. The United Nations secretary general condemned the use of force.

Video showed officers of unknown origin approaching a crowd late Tuesday in an upscale suburb called Lekki. People sang the Nigerian national anthem as shots rang out.

Babajide Sanwo-Olu, the Lagos state governor, described it as one of the “darkest hours from our history as a people” on Wednesday but said authorities had recorded only one fatality at the hospital. It was “an isolated case,” he tweeted.

Thirty were injured, said Sanwo-Olu, who added the government will investigate the shooting.

The Nigerian president did not address the bloodshed in a Wednesday statement, appealing instead for calm and patience as police reforms “gather pace.”

The military said there were no soldiers at the scene of the violence, tweeting on its official account: “Fake News!!!” (A military spokesman did not respond to requests for comment.)

Protesters and human rights groups, as well as the head of the United Nations, disputed the official record on Wednesday.

“All I heard was bullets — left, right and center,” said a 37-year-old man, who recorded a live Instagram video at the demonstration in front of the Lekki toll plaza.

At least four people around him died, said the man, who spoke on the condition of anonymity because he feared retribution.

“There was no warning,” he said, breaking down in tears.

He ran through the night to escape, he said.

Hundreds had gathered that afternoon at the normally busy toll plaza in defiance of the curfew, dancing to West African pop music in the street, numerous videos show.

Camille Shaiyen, a 28-year-old bank employee, briefly stopped by to hand out rice and chicken.

“It was calm,” she said. “There was a team of people cleaning. There were people helping with parking.”

Some protesters held umbrellas to shield them from the sun. Many waved Nigerian flags, believing it would protect them from security forces.

“Sit down,” a woman advised the crowds, according to footage from the scene. “Don’t run. Raise your flag. They won’t attack you.”

Just after 7 p.m., however, uniformed men surrounded the crowd. Video captured the streetlights going out. Gunfire erupted. Then screams.

“Nothing in my life could have prepared me for what I saw,” said Shaiyen, who later watched live streams on Instagram.

Amnesty International said in a news release Wednesday that its investigation found 12 protesters had been killed by Nigerian security forces Tuesday in Lekki and Alausa, another part of Lagos.

At least 56 people have died across Nigeria since the protests started, Amnesty found.

Nationwide protests erupted on Oct. 7 after video emerged of officers from the federal Special Anti-Robbery Squad, or SARS, shooting an unarmed man.

Celebrities including Rihanna, Cardi B, Burna Boy and the Nigerian soccer star Odion Jude Ighalo have rallied behind the cause, helping the hashtag #EndSARS go viral.

U.S. Democratic presidential candidate Joe Biden urged Buhari and the Nigerian military to end the “violent crackdown.”

“The U.S. must stand with Nigerians who are peacefully demonstrating for police reform and seeking an end to corruption in their democracy,” he said in a statement Wednesday.

SARS was formed nearly three decades ago to fight violent crime, including kidnapping and robbery.

The officers, who often worked in plain clothes, developed a reputation for stopping young people, accusing them of theft and then stealing their money, activists say. The Nigerian Police Force said it has fired at least two officers since the demonstrations began and has said it would probe past atrocities with the help of civil society leaders.

Amnesty International said it has documented 82 cases of the squad’s abuses in the past three years, including beating, hanging, mock execution, sexual assault and waterboarding.

In Lagos, chaos escalated on Tuesday following news reports that officers had killed two people. A crowd torched a police station, and protesters blocked roads.

Police deployed anti-riot officers to “protect the lives and property of all Nigerians and secure critical national infrastructure across the country,” the department said in a statement.

The crisis intensified as Nigeria grapples with the economic shock of the coronavirus pandemic on top of an Islamist insurgency in the northeast — both of which have fueled resentment among its 200 million people.

“Sustaining our democracy thus far has been a collective struggle,” Buhari said in a May speech commemorating 21 years of uninterrupted civilian leadership.

Now critics warn that stability is at stake, saying Tuesday’s shooting reminded them of the nation’s three-decade run of juntas and coups.

“It was reminiscent of military-run Nigeria,” said Bulama Bukarti, a Nigerian lawyer in London who represents victims of police brutality.

Previous scandals have eroded public trust in the nation’s security forces.

Nigerian soldiers drew international condemnation in 2018 for shooting at protesters in Abuja.