MANCHESTER, United Kingdom (AFP) - The sounds of happy cheering and bouncy pop were still ringing in the ears of excited young fans of US singer Ariana Grande when an altogether more menacing noise erupted.

Within minutes, police in Britain's third city of Manchester knew they had a major incident on their hands after a bomber targeted the youngsters leaving the performance in one of Europe's largest indoor venues.

The investigation has reportedly recovered CCTV footage showing the suspected bomber, Salman Abedi, striding into the foyer of the Manchester Arena at 10.30pm on Monday and detonating a home-made explosive device.

Panic ensued both in the foyer and inside the 21,000-seat auditorium as fans, and parents waiting to pick up their children, fell over each in a stampede to flee.

At least 22 people were killed - one girl aged just eight - and more than 50 others were wounded, many seriously. Twelve of them were aged under 16.

Abedi, 22, also died in Britain's worst terror attack since the transport network in London was hit by suicide bombers in July 2005.

'APPALLING TERRORIST ATTACK'

Greater Manchester Police had labelled it a possible act of terror within 17 minutes of the blast going off, mobilising a major operation that included the closure of a nearby transport hub and roads.

They were flooded with 240 phone calls within the first hour of the incident, and went on to mobilise more than 400 officers and staff during the night.

The police carried out a controlled explosion near the arena at 1.35am, on what turned out to be a pile of abandoned clothing.

By 2.15am, Prime Minister Theresa May was acknowledging an "appalling terrorist attack" and she went on to brief opposition Labour leader Jeremy Corbyn in the kind of contact that only takes place during times of national emergency.

Grande, a former child TV star, tweeted at 3.51am that she felt "broken" by the bloodshed, as the future of her current world tour was plunged into doubt.

The coming day would bring heartbreak as parents grieved for young lives horrifically cut short, or issued poignant appeals for those still listed as missing.

The day would also bring questions: about security at the venue, about Abedi's background, and about the implications for a country heading to the polls on June 8 for a general election in which national security had been only a fringe issue.

But defiance sounded the loudest note, as a vigil on Tuesday evening outside Manchester's imposing Town Hall erupted in chants declaring that the city would prevail over terrorism.