A 23-year-old man was arrested Tuesday morning in connection to a probable suicide bombing at an Ariana Grande concert that left 22 people killed and dozens more injured in Manchester, England.

Chief Constable Ian Hopkins of Greater Manchester Police Department said early Tuesday that 22 people are confirmed dead, with around 59 other casualties.

“We are currently treating this as a terrorist incident until we have further information,” Chief Constable Hopkins said.

He also said that the priority for police Tuesday is to establish whether the suspected bomber, named as 22-year-old Salman Abedi, was acting alone or as part of a network, The Associated Press reported.

The head of Manchester’s police said local police were working with counter-terrorism officials and solicited tips to an anti-terror hotline.

Hours after the attack, which happened outside the arena as thousands of people were streaming outside upon the concert’s conclusion, police arrested a 23-year-old man in South Manchester in connection to the attack.

Britain had an election set for June 8, but the major parties said they would suspend their campaigns for the moment as the nation went into mourning.

Prime Minister Theresa May, a Conservative, said “all our thoughts are with the victims and the families of those who have been affected.”

Ms. Grande’s music is pitched to younger audiences and numerous eyewitness accounts said many of the casualties were children and teenagers, along with parents arriving to pick them up as the concert concluded.

The first victim was identified as Georgina Callander, a student at the Bishop Rawstorne Church of England Academy in Croston, northwest of Manchester, AP said. Police later identified an 8-year-old girl, Saffie Roussos, among the 22 killed in the bombing.

An ambulance official said 12 children under the age of 16 were among 59 injured in the attack, the wire service reported.

Witness Gary Walker, from Leeds, England, told Britain’s Daily Mail that he was with his wife in the foyer of the arena waiting to pick up his two daughters who attended the concern.

“We heard the last song, and quite a few people were flooding out and then suddenly there was a massive flash and then a bang, smoke,” Mr. Walker said. “I felt a bit of pain in my foot and my leg. My wife said, ‘I need to lie down.’ I lay her down, she’d got a stomach wound and possibly a broken leg. I was about three meters from the actual explosion. I am surprised I got away so lightly”

Tim Farron, leader of Britain’s Liberal Democratic Party, called the blast “a shocking and horrific attack targeting children and young people who were simply enjoying a concert,” said

Besides calling it a “direct attack on children,” Nigel Farage, a former leader of the U.K. Independence Party and a frequent critic of immigration and the threat of Islamist terrorism, also predicted that the national debate around those subjects would change.

“This is going to be a very big shock for the country when it wakes up tomorrow morning,” he said on Fox News.

Manchester Arena, which was hosting the concert, issued a statement saying the explosion occurred on the street outside the venue.

“We can confirm there was an incident as people were leaving the Ariana Grande show last night,” read the statement. “The incident took place outside the venue in a public space.”

Some previous reports indicated that the blast occurred in the foyer of the arena near the box office, though people there would have been injured or killed as they left the concert and would have been affected by the resulting panic.

Concertgoer Abby Mullen from Airdrie, Scotland, told the Daily Mail that “as we were leaving, a bomb or explosion went off centimeters in front of me. People’s skin and feces were everywhere including in my hair and on my bag.”

“That sound, the blood and those who were running around clueless with body parts and bits of skin missing will not be leaving my mind anytime soon or the minds of those involved,” she said.

Scores of gun-toting police — a rare sight in Britain — were patrolling the area. The Victoria railway station, which serves the arena and abuts the explosion site, was closed down and will remain so Tuesday.

Video posted from inside the concert hall by the official Twitter account of Miss Grande’s tour shows panicked concertgoers reacting to a loud bang.

Concertgoer Evie Brewster told the Daily Mail newspaper that “there were thousands of people trying to get out at once. They were all screaming and crying. The whole place smelt smokey and burnt.”

The singer already had left the arena and was not among the casualties, her publicists said.

Ms. Grande said on Twitter that she was “broken. from the bottom of my heart, i am so so sorry. i don’t have words.”

The 21,000-seat Manchester Arena, which opened in 1995, is one of the largest indoor venues in Europe.

Condolences from across the U.S. poured in on social media for the victims.

The Boston Police Department, which dealt with twin terrorist bombings at the 2013 Boston Marathon, said on its Twitter account, “Our thoughts and prayers are with those hurt, harmed or injured at the Ariana Grande Concert in Manchester, England this evening.”

Rep. Justin Amash, Michigan Republican, said on Twitter that he was sending “prayers for the people of Manchester and the United Kingdom — especially for those killed or injured and their families.”

Rep. Adam B. Schiff, California Democrat and a member of the House Permanent Select Committee on Intelligence, said it was “devastating to hear about carnage at concert in Manchester.”

“Is there no limit to the willingness of people to massacre fellow human beings?” he said on Twitter.

In addition to Mr. Farage, White House official Sebastian Gorka also broached the possibility of Islamist involvement.

He said on Twitter that the Manchester killings could have been timed to coincide with the date of another fatal attack in the U.K. in 2013.

Mr. Gorka tweeted, “Manchester explosion happens on 4th anniversary of the public murder of Fusilier Lee Rigby. Dates matter to Jihadi terrorists.”

Mr. Rigby was a British Army soldier stabbed and hacked to death in London in 2013 by two Muslims, a slaying that also took place on May 22.

There was no immediate official reaction from President Trump, who was in Israel in the midst of a nine-day trip to the Middle East and Europe. Mr. Trump is scheduled to meet Tuesday with Palestinian Authority President Mahmoud Abbas in the West Bank.

The U.S. Department of Homeland Security warned Americans that they “may experience increased security” in public places as a precaution. But the agency said it has “no information to indicate a specific credible threat involving music venues in the United States.”

Manchester police found a “second suspect device” at nearby Cathedral Gardens and conducted a controlled explosion early Tuesday morning, but they said it turned out to be a sack of clothes, “not a suspicious item.”

• Andrea Noble contributed to this report.

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