Scooter said on the micro-blogging website: "Words can not express our sorrow for the victims and families harmed in this senseless attack. I'm sending all my love".
Police have appealed for concert-goers and witnesses to provide them with any footage they have from the scene if they believe it can assist the probe.
British Prime Minister Theresa May says that it is "beyond doubt" that Britain and the city of Manchester have fallen victim to "a callous terrorist attack". "We ask all of you to hold the victims, their families, and all those affected in your hearts and prayers".
"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-haired girl with flowers in her hair.
"Broken", the singer tweeted, hours after news broke of the deadly incident. One wrote: "May they taste what the weak people in Mosul and (Raqqa) experience from their being bombed and burned", according to the US -based SITE Intelligence Group.More news: World leaders condemn Manchester attack, express condolence
This has not been confirmed, but Manchester police said that it's "currently being treated as a terrorist incident until police know otherwise".
Musicians such as Ed Sheeran, Nicki Minaj and Katy Perry all offered supportive tweets.
In March, a British-born convert to Islam plowed a vehicle into pedestrians on London's Westminster Bridge, killing four people, before stabbing to death a police officer who was on the grounds of parliament.
Police have set up a hotline for people affected by the incident: 0161 856 9400. The government was expected to call an emergency Cabinet meeting.
Police said roughly 50 people were injured.More news: Turkey summons United States ambassador over 'aggressive' acts against bodyguards during Erdogan visit
There was mass panic after the explosion at the end of the concert, which was part of Grande's The Dangerous Woman Tour.
According to the website, Grande is "in hysterics" after the bombing.
This is a fast-moving investigation and we have significant resources deployed to both the investigation and the visible patrols that people will see across Greater Manchester as they wake up to news of the events last night.
The downtown area itself, a mix of grand Victorian statement architecture and recently regenerated and contemporary buildings, is a popular nightlife destination, that draws in thousands from across the Greater Manchester area each evening.
There were no immediate details of what happened during the concert by the American singer.More news: South Korea says fires warning shots at object flown from North Korea
Video from inside the arena showed people screaming as they made their way out amid a sea of pink balloons.
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