Police said at least 22 people were killed and dozens injured, who are being treated in six hospitals.
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"Families and many young people were out to enjoy a concert at the Manchester Arena and have lost their lives".
He added: "This is an attack on innocents".
"What I can confirm is that there are children among the deceased".
The city's regional government and its mayor, Andy Burnham, were among scores of Twitter users who circulated the MissinginManchester hashtag, used by people looking for family members and friends.
May's Conservatives will suspend campaigning, broadcaster Sky News has reported.
"We are now treating this as a terrorist incident until we have further information", Hopkins said, as aired by the Sky News broadcaster.
Britons are due to go to the polls on June 8.
There was panic after the explosion, which struck around 10:30 p.m. (2130GMT) Monday night as Grande was ending the concert, part of her Dangerous Woman Tour.
Some initial reports said a speaker or a balloon had exploded, but it quickly emerged it was something far more sinister.
Taxi services offered to give stranded concertgoers rides home for free, and residents opened their homes to provide lodging for people who could not get home because public transport had shut down.
City officials said the true spirit of Manchester was surfacing in the hours after the incident.
DHS says it is closely monitoring the situation at Manchester Arena and working with United Kingdom officials to obtain additional information about the cause of the explosion.
The government is urging USA citizens in Manchester to heed directions from local authorities and be vigilant about their security. Police say they are treating as a terrorist attack.
"That was going into the concert, so there was no security whatsoever on the outside where the explosion took place".
The 23-year-old Grande, true to her youthful fan base, is a social media phenomenon with 105 million followers on Instagram and 45.6 million followers on Twitter. She said her phone had not stopped ringing since her tweet.
Taylor Swift tweeted, "My thoughts, prayers and tears for all those affected by the Manchester tragedy tonight. I'm sending all my love".
Hopkins said the police department is working with local and national agencies in their investigation.
Delgado said: "There were people on the floor ..." Police said there are 19 confirmed deaths.
British Prime Minister Theresa May said the incident was being treated as a terrorist attack.
"I wish I could say that I am ok, but I'm not", she wrote.
The government is expected to call an emergency cabinet meeting to deal with the incident.
Police said they were called at 2133 GMT to reports of an explosion at Manchester Arena at a concert by pop star Ariana Grande, who is popular with teenagers.
Police said roughly 50 people were injured.
Emergency vehicles were helping the injured and bomb disposal units were later seen outside the venue.
In a statement released to the press, Grande's manager Scooter Braun updated fans, well wishers, and would-be concertgoers all over the world about the status of The Dangerous Woman tour.
Hayley Lunt was staying at a hotel nearby and had taken her 10-year-old daughter Abigail to her first concert at Manchester Arena on Monday evening. James said he believed he heard a second explosion as they were running out.
Grande said she had been left "broken" by the events. After Manchester, Grande was to perform at venues in Europe, including Belgium, Poland, Germany, Switzerland and France, with concerts in Latin America and Asia to follow.
There were no immediate details of what happened during the concert by the American singer.