Paramedic Barry Timms said: "There was a very large crowd, believed to be 21,000 people, in attendance and there had been a crush outside the event trying to get in as JLS started playing.
"A metal fencing collapsed trapping a couple of people underneath. One of them was a female in her 30s that's got some quite serious injuries and a teenage girl also has crush injuries and was taken to hospital.
"As the barrier collapsed it's believed people were trapped and trampled underneath it."
He said a young boy and another woman were also taken to hospital with fractures to arms and legs.
Mr Timms said the woman with more serious injuries is being treated and it is hoped she will be fine.
The other three had minor injuries and would be discharged, he added.
Martin Krol, who lives in a flat overlooking Millennium Point, said he saw people pushing at a fence at the front of the crowd.
He said: "Police had already stopped people from going through into the main bit because there were so many of them, so an extra line of people had been created at the back of the main crowd.
"The police were totally outnumbered - there were far too many people in the crowd for them to be able to cope.
Eyewitnesses describe the crush
"It was absolute chaos - the ambulances couldn't get through - and it took quite a while for them to be able to clear the area because there were so many people.
"We saw people who had been knocked unconscious and carried out into ambulances. One person was carried out on a spinal board - and we saw a police officer injured too."
The area around Millennium Point has now been cleared and emergency crews were urging people to stay away from the area.
A council spokesman said the gates were closed after the square reached its capacity.
An emergency meeting was held between the council, emergency services and the joint organisers, radio station BRMB, and a decision was made to cancel the event.
Steve Hollingworth from Birmingham City Council said the event had been properly organised.
He told BBC News: "We had a full meeting beforehand with all the emergency services. Everybody was comfortable with what we were doing.
"We are experienced at holding events but clearly the crowd was very significant and the people outside the event were clearly determined to get in.
"We'll do a full investigation and learn from what's happened."
JLS first made their name as runners-up in the fifth series of ITV reality talent show The X Factor in 2008.
This year, their first two singles, Beat Again and Everybody In Love, have shot to number one in the charts and the band won two Mobo (Music of Black Origin) awards.
They also attract a legion of particularly fervent young female fans. In December, the band staged a free show at Croydon's Fairfield Halls and five teenage girls were injured as more than 2,000 fans turned up.
Other acts that had been scheduled to appear as part of the five-hour event in Birmingham included Calvin Harris, Tinchy Stryder, Natalie Imbruglia, Chipmunk, Girls Can't Catch, Little Boots, The Saturdays, The Noisettes, Taio Cruz and Pixie Lott.
The night was due to finish with a fireworks display.
Prior to the event, a statement on the city council website said: "In order to keep the area safe for everyone West Midlands Police have asked people not to bring their own alcohol, and not to bring valuables with them as a precaution.
"They hope people will refrain from any pushing and shoving, so that people of all ages can enjoy the concert comfortably."
The city's Christmas lights are now due to be switched on in Birmingham's High Street at 1630 GMT on Sunday.
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