Gold for Simmonds
Ellie Simmonds claimed ParalympicsGB's second gold medal in the pool when she triumphed in the S6 400 metres freestyle.
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She held off the challenge of Victoria Arlen and in the process demolished the American's world record by more than five seconds.
After the race, an emotional and "exhausted" Simmonds choked back tears as she paid tribute to the crowd. She said: "It was so tough.
"It was so hard on the last 100 and I thought I have to put my head down and I will do it for everyone who has supported me.
"Everyone has been wishing me good luck and I did this for myself, my family and everyone."
Thousands also cheered her on at the big screen in the Olympic Park, which erupted in a cacophany of noise at the end of the race.
Her victory in five minutes 19.17 seconds followed the gold won by Jonathan Fox on the opening night and came immediately after Simmonds' Swansea team-mate Matthew Whorwood secured bronze in the previous race in the men's equivalent.
The crowd at the Aquatics Centre had grown increasingly more raucous as Whorwood made his way to the bronze in the previous race and on Simmonds' introduction the noise was intense.
The pair were neck and neck going into the final turn with the Briton 0.08 secs ahead and as she pulled away, the American had no answer as Simmonds touched first to send the crowd into a frenzy.
It was only at 2pm that Arlen's participation in the race was confirmed in a statement released by the International Paralympic Committee.
Earlier this week the American swimmer had been deemed ineligible for competition and excluded from London 2012.
The United States' appeal to the IPC was upheld and the 17-year-old was reinstated on Thursday.
In a two-stage process, Arlen passed an initial assessment but she was also observed during racing today and it was only following this morning's heat that any doubt surrounding her involvement in her three S6 freestyle events was dispelled with a review in a year's time.
A swimmer's classification can change for different strokes because the nature of their impairment may affect their ability to perform a particular stroke. This applies to Arlen who competes in the SB5 class in breaststroke in which she will continue to be observed.
Simmonds struggled to keep her emotions in check as she came to terms with her achievement.
"I need to sort myself out because I don't want to cry again," she told Channel 4. "I just went out there and gave it my all.
"I'm exhausted but I can't wait to see my coach (Billy Pye).
"I'm exhausted. I can't believe I did it.
"It was so tough. I saw her (Arlen) on the last 100 and I was like, 'I'm going to have to put my head down' and I thought, 'I'm going to do it for for everyone who's supported me'."
Asked whether the uncertainty surrounding Arlen's participation had affected her, she said: "Not really. I think it pushed me even harder.
"I knew that she was on it tonight, I knew I had to go there and give it everything, but 5.19 - my coach said it was going to be won in a 5.19 but I didn't believe him.
"I just put my head down and gave it everything. I was exhausted but I just put my head down and went for it.
"Everyone was wishing me good luck which was so nice. I did it for myself, I did it for my family."
With the medley still to come, she added: "I'm on form. I've just done a six-second PB so everything's going really well so I'm just going to go back, enjoy this moment, enjoy my medal and my presentation. I'm excited."