Earlier today, at least 19 people were killed and a further 50 injured following a terrorist attack at Grande's concert in Manchester.
Teenage screams filled the arena just after the explosion, which also killed the attacker and injured dozens.
British counter-terrorism police have said they are making on average an arrest every day in connection with suspected terrorism.
Confirming the details of the attack, Chief Constable of Greater Manchester Police Ian Hopkins said the investigation to identify the perpetrators behind the deadly attack is underway, until which the attack is being treated as a terrorist incident.
A police helicopter hummed over the city as somber commuters hurried to work.
Manchester Arena said the explosion happened outside the venue, as people began streaming from the doors.
The concert was attended by thousands of young music fans in northern England. Calvin Welsford, an 18-year-old concertgoer, said he heard a "loud bang" a couple of minutes after Grande's set had finished.
Jenny Brewster said she was leaving the concert with her 11-year-old daughter when the blast hit.
She said the explosions rang out as soon as Grande left the stage. "It was nearly like they waited for her to go". "It was chaotic. Everybody was running and screaming and just trying to get out". "What should have been a superb evening is now just frightful".
Campaign events ahead of the June 8 general election will now be put on hold as Britain comes to grips with the incident and its aftermath. Police cars, bomb-disposal units and 60 ambulances raced to the scene as the scale of the carnage became clear. More than 400 officers were deployed.
NY authorities on Monday stepped up security at sensitive sites after a deadly blast at a concert in Britain, although officials said there was no indication of a threat to U.S. targets. "We heard the last song go and then suddenly there was a massive flash and then a bang and smoke", Walker was quoted as saying.
Home Secretary Amber Rudd decried "a barbaric attack, deliberately targeting some of the most vulnerable in our society - young people and children out at a pop concert". The local ambulance service says 59 people were taken to hospitals. "My thoughts are with families and friends of those who have died and been injured".
Among the names being circulated was Olivia Campbell. "She's registered as a missing person because she hasn't shown up at any hospital yet", Charlotte told the BBC.
"Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn't answering her phone, pls help me", said one message posted alongside a picture of a blonde girl with flowers in her hair.
The department says the US public may experience increased security in and around public places and events. She said her phone had not stopped ringing since her tweet. "Anyone seen my daughter Olivia Campbell", she wrote.
There was no immediate claim of responsibility, but US officials drew parallels to the coordinated attacks in November 2015 by Islamist militants on the Bataclan concert hall and other sites in Paris, which claimed about 130 lives. "And I mean completely obliterated", he added.
If confirmed, it would be the deadliest militant assault on Britain since four British Muslims killed 52 people in suicide bombings on London's transport system in July 2005. "Everyone was screaming and running, there were coats and people's phones on the floor".
Up and coming American popstar Sabrina Carpenter (@SabrinaAnnLynn), who recently played in the northern English city with British pop rockers The Vamps, expressed her shock.
The Dangerous Woman tour is the third concert tour by 23-year-old Grande and supports her third studio album, "Dangerous Woman".
Grande's role as Cat Valentine on Nickelodeon's high school sitcom "Victorious" propelled her to teen idol status, starting in 2010.
Manchester Arena, the largest indoor arena in Europe, opened in 1995 and is a popular concert and sporting venue.