The British Prime Minister Theresa May said the incident was being treated as a terrorist attack.
The incident took place late on Monday night following a pop concert by the US singer Ariana Grande.
Two US officials quoted in a report by Reuters have said the explosion was likely triggered by a suicide bomber.
A witness who attended the concert told Reuters she felt a huge blast as she was leaving the arena, followed by screaming and a rush by thousands of people trying to escape the building.
No individual or group has so far claimed responsibility behind the incident so far.
This comes as the Islamic State of Iraq and Syria (ISIS) terrorist group had claimed responsibility behind an attack in London city late in March this year which left at least five people dead.
The incident took place near the Houses of Parliament on 22nd March after a suspected terrorist drove a car over Westminster Bridge, near the Houses of Parliament, mounted the pavement, and began hitting pedestrians indiscriminately.