Here's a timeline of events for the attack:
What we know about the attack
The wounded and the missing
The wounded are being treated in several different area hospitals and some of them are battling life-threatening injuries, authorities said.
Many people are still missing and an emergency hotline has been set up for people concerned for “loved ones who may not have returned home," police said. Facebook activated its global "safety check" feature in the wake of the deadly attack.
On Tuesday, Manchester police also announced the arrest of a 23-year-old suspect in connection with the attack.
So far, they have not said if there were any accomplices in planning or carrying out the attack or if any suspects have been linked to terrorist organizations.
The terror group ISIS claimed responsibility for the attack on Tuesday in an Arabic-language news release.
“The priority is to establish whether he was acting alone or as part of a network,” Ian Hopkins, chief constable of Greater Manchester Police, said of the attacker at a press conference Tuesday.
“Our priority is to work with the national counterterrorism policing network to establish more details about the individual who carried out this attack," he added.
Police have not identified the nationality of the person behind the attack, but a former director of the security service M16 told BBC earlier that the bomber was likely already known to security services.
Prime Minister calls the attack "sickening" after an emergency meeting on Tuesday
U.K. Prime Minister Theresa May called the suspected terrorist attack "sickening" and "appalling." She held an emergency meeting with government ministers, military and security chiefs to plan the government’s overall response.
The prime minister announced Tuesday evening that the country's threat level had increased to critical, meaning another attack is imminent.
May said the attacker deliberately targeted children and young people "who should have been enjoying one of the most memorable nights of their lives."
"We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherished but as an opportunity for carnage," May said.
The concert venue, which holds about 21,000 people, is one of Europe’s largest indoor arenas, according to its website, and it is connected to the city’s second-largest train station.
Ariana Grande's tour
It’s not clear if Grande, who has two concerts scheduled in London later this week, will go on with her world tour as planned.
The pop star‘s "Dangerous Woman" tour was originally scheduled to end on Sept. 21 in Hong Kong.
Grande was not hurt in the attack, but she tweeted last night that she was feeling “broken.”
ABC News' Emily Shapiro contributed to this report.