They said it was not immediately clear if he acted alone.
Britons are due to go to the polls on June 8. "As we came outside to Victoria Station there were just people all over the floor covered in blood".
The identities of those killed or injured have not been confirmed, and no arrests have been made. Some were treated on site while others were taken to local hospitals.
Others offered rides and rooms to those in the area using the hashtag #RoomforManchester.
In a statement, Manchester mayor Andy Burnham described the tragedy as a "terrible night for our great city".
The Department of Homeland Security says there is no evidence of credible threats against music venues in the USA, as England reels from an explosion at an Ariana Grande concert late Monday. "However, the public may experience increased security in and around public places and events as officials take additional precautions".
As of this writing, no cause for the explosions has been confirmed by officials.
The DHS also urged United States citizens in the area to pay heed to the directions from the local authorities and maintain security awareness.
The explosion killed at least 19 people and injured dozens.
Ariana Grande, pictured in a file photo above, is a 23-year-old singer from the US.
Robinson took dozens of teenage girls to the nearby Holiday Inn Express hotel and tweeted out her phone number to anxious parents, telling them to meet her there.
"Our thoughts and prayers go out to the victims", it said.
"I'm heartbroken over what happened in Manchester tonight Sending love to everyone involved", Harry Styles tweeted.
It was used by friends or family members of missing loved ones to try to locate them following the incident.
Police were called to reports of an explosion at the Manchester Arena at 10.33pm, shortly after USA singer Ariana Grande had finished her performance. The government's emergency Cobra committee is scheduled to meet Tuesday morning.
May says the government is trying to establish "the full details of what is being treated by the police as an appalling terrorist attack".
Earlier winger Jesse Lingard said: "My thoughts and prayers are with everyone affected by last nights attack on this handsome city".
Emergency vehicles were on the scene helping the injured and bomb disposal units were later seen outside the venue.
Eyewitness Sasina Akhtar told the Manchester Evening News there was an explosion at the back of the arena as Grande performed her final song.
Police officers stand next to their vehicles near the Manchester Arena after a blast at Ariana Grande concert Tuesday May 23, 2017.
Greater Manchester Police have set up a help centre for anyone needing assistance at Gate 11 of City's Etihad Stadium.
There were few immediate details and trains into the area were suspended.
"In the absence of conclusive evidence, the choice of venue, the timing and the mode of attack all suggest this was terrorism", said a US counter terrorism official who also spoke on condition of anonymity.