Bomb disposal experts were trying to disarm the bomb; police were setting up roadblocks and broadcasting warnings from a helicopter, but there just wasn't time.
So big was the bomb, so dense the network of streets and so thick were the crowds of shoppers on the hottest Saturday of the year that many were injured and damage was widespread. Glass rained from the sky and terrified people ran for safety.
Dozens were cut by glass and hundreds had narrow escapes. Here are some of their stories:
RACHEL REEVES, 18, from Stockport, was working in the WH Smith branch in the Arndale Centre. "There was a coded warning about 10.10am but they weren't too worried about it. But later the warning came through as a 555, which meant we had to evacuate the whole building. We got as far as Piccadilly and it seemed that the people working in the shops up there didn't know anything about it. They were still carrying on as normal.
"We were on King Street when the explosion went off and glass shattered all around us. A few people got hit by glass and were cut, and later there were about four people who we saw injured by flying glass.
"We thought the explosion had gone off right next to us. I couldn't believe it when I was told it was close to the Arndale because that is almost half a mile from where we were. We headed further out and when we got to Albert Square we heard there would be another one, so we got right out of town."
MALCOLM CROFT, a photographer, was a quarter of a mile from the Arndale Centre when the explosion happened. "It was a big blast. All the windows in the shops were blown out and I saw people hit by flying glass.
"There was a lot of screaming and panic. People began rushing for shelter and shielding their faces.
"A police helicopter had been hovering over us with a loud-speaker message warning us to keep away from the Arndale Centre. The blast happened as the area was being cleared."
SANDRA EAVES, of Chorlton-on-Medlock, Greater Manchester, was in the city centre shopping with her husband and daughter. "When we got into Piccadilly it was cordoned off but nothing was happening. Police started moving everyone back and we went into Superdrug in Piccadilly Gardens when there was an almighty bang.
"The place shook and everybody seemed to be waiting for the place to cave in, it seemed that close. We were told to evacuate the building but we didn't know where to go.
"It seemed an unreal situation because there was no panic. People were crying because they were shocked, but most people were walking away quite calmly.
"We started to walk out towards London Road and we saw that the glass had been blown out of shops and offices almost to Piccadilly train station.
"People were queuing at telephone boxes to phone home and one girl told us that policemen had told her to leave the city as soon as possible so we did."
MICHAEL DOHERTY, 38, a builder, suffered a badly gashed arm. "I just heard a bang. I knew it was a bomb straight away. I was hit by flying glass. It was like a white shower all around us. I feel very lucky."
PAUL GILLESPIE, 20, is a fellow builder. "We all just dived on top of each other. The police had cleared all the roads around us and they told the crane driver to come down but that was it. We were all lucky we were wearing hard hats."
MIKE DAVISON, 46, a television engineer who lives in Dortmund, Germany, was arriving in Manchester by coach on a holiday trip. "The bus was passing some high buildings near the bus station. They were old buildings and they seemed to shake visibly. I thought they were going to come down on top of the coach. It was a terrific bang and there was dust flying everywhere.
"There are a lot of foreigners in the city who have spent good money to come here and enjoy themselves only to find there are bombs going off."
PAUL BENNETT, 26, a music student at the Royal Northern College in Manchester, suffered a gash on the arm. "I was up near Lewis's department store. We were away from where the bomb went off. We were walking along then all of a sudden there was this massive bang.
"It was like something out of a Hollywood movie. Glass was flying everywhere and there were people running and screaming all over the place. Then what looked like a huge sheet of glass came crashing down over my head. I grabbed my girlfriend to protect her but a shard went straight through my arm."
PHILLIPE TOURNEFIER, a French football fan in England for Euro 96, found himself tending to a German fan with multiple cuts made by falling glass. "This sucks. These people are disgusting. We have come to England to enjoy ourselves and to have a good time. We were having a wonderful visit to Manchester, we enjoy the matches and the weather is brilliant. Then this happens. I cannot bear to see it. It is a disgrace."
ADRIAN RHODES, chaplain at Manchester Royal Infirmary, dealt with distressed casualties. "On the whole I've been very surprised how calm people have been and how well they have coped with it.
"There are some very shocked people and some very game elderly people and also some very frightened people.
"I wish the people who have done this could see the faces of some of the people I've had to deal with today - some of the shocked children and elderly people - and then explain why it was necessary."Reuse content