In a statement, Greater Manchester Police said they were called to the Manchester Arena in England just before 10:35 p.m. local time on Monday. Authorities are telling people to avoid the area.
At the moment, the incident is being treated as a "terrorist incident until police know otherwise," police said.
U.S. law enforcement officials have been informed that the leading theory is that the Manchester Arena incident was the work of a suicide bomber, though they caution that this is preliminary information, according to a senior law enforcement official briefed on the investigation.
The wounded are being treated at six different hospitals, Greater Manchester Police Chief Constable Ian Hopkins said in a press conference. "We're doing all that we can ... as we gather information about what happened," he said, asking people to remain vigilant.
The explosion happened inside of the arena’s foyer area after the concert finished, according to witnesses, who reported hearing a loud bang as they exited the arena. The venue holds about 21,000 people and is one of Europe’s largest indoor arenas, according to its website.
The concert arena is connected to the Manchester Victoria Station, the city’s second-largest train station.
The British Transport Police department said extra officers would be patrol at "key railway stations as well as on trains around the country” in the wake of the attack.
"Throughout the day, commuters can expect to see additional officers on their journey,” BTP Assistant Chief Constable Robin Smith said in a statement early Tuesday. "This will include both armed and unarmed officers.”
“As the public would expect in response to an incident such as this, extra BTP officers will be on patrol at key railway stations as well as on trains around the country,”
U.K. Prime Minister Theresa May and Jeremy Corbyn have agreed that all national campaigning in the General Election were "suspended until further notice" in the aftermath of the incident, according to the Press Association, which cited Labour officials.
May is set to hold an emergency COBRA meeting at 9 a.m. local time. The group typically meets after major incidents to plan the government’s overall response. The group -- composed of government ministers, military and security chiefs -- will also review the threat level, which is currently one below the highest possible level.
Concertgoers reported hearing a loud bang at the end of the concert and attendees rushed toward the exit.
Eyewitness Karen Ford told the BBC, "The lights had come up everyone was just getting out and walking towards the stairs, when all of a sudden this huge sound, which sounded like an explosion went off."
She added: "Everyone just stopped and turned around, and then somebody shouted 'it’s a bomb' and everyone just started running. Everybody was trying to push people up the stairs. There was a lot of children there without parents. There was no one to calm them down so everyone was just screaming, crying and pushing."
Andy Holey, an eyewitness, told the BBC that he was waiting outside the concert and was blown over by the force of what he described as an "explosion." When he awoke, he said he saw many casualties around him. He added that it was unclear if they were injured or dead.
Abby Barker, who attended the concert, told ABC News that Grande had just left the stage when there was a loud bang. "I looked across the arena and everyone over there started running in different directions, screaming in panic," she said. "We all started panicking too and ran out the doors and ran down the stairs out of the arena. We got outside and children were crying their eyes out, people talking about it being a bomb/gunshots, there were many parents running towards the arena but no one knew exactly what it was."
Emergency personnel treated some of the injured at the nearby Manchester Victoria railway station.
A rep for Grande confirmed that she was not harmed.
A distraught Grande tweeted that she was "broken."
In a statement, Grande's manager, Scooter Braun, echoed her tweet, writing, "Our hearts are broken."
In a statement, Manchester Arena confirmed that there had been an incident but said it occurred outside the venue. "Our thoughts and prayers go out to the victims," it tweeted.
Manchester Victoria station has been evacuated and trains are unable to run.
According to a spokesman for the British Home Office, they are holding a senior staff meeting on the Manchester incident this evening.
U.K. Prime Minister May said in a statement that they "are working to establish the full details of what is being treated by the police as an appalling terrorist attack."
"All our thoughts are with the victims and the families of those who have been affected," she added.
The U.S. State Department said that it was monitoring the situation closely.
"Our thoughts and prayers are with the family and friends of those killed and injured in the incident," a State Department official said. "The safety and security of U.S. citizens overseas is one of our highest priorities. The U.S. Embassy in London is monitoring the situation closely."
ABC News' Josh Margolin contributed to this report.