South Africa's Oscar Pistorius
made history by becoming the first amputee sprinter to compete at the Olympics.
The four-time Paralympic champion, 25, whose legs were amputated below the knee as a baby, finished second in his 400m heat in a time of 45.44 seconds to reach Sunday's semi-final.
"I didn't know if I should cry or be happy. It was such a mix of emotions," Pistorius told BBC Sport.
American defending champion
pulled up injured.
who failed a drugs test in 2009,
appeared to be still suffering with an Achilles tendon problem that affected him earlier in the season.
He stopped running after 250m and crossed the line at walking pace.
"It's Olympic year - I tried to go for it, and it just didn't happen," he said.
Pistorius was impressive in his heat, coasting over the line behind Luguelin Santos of the Dominican Republic, who won in 45.04.
"Thank you so much to everyone who has supported me," he said. "Thank you so much to my family. I saw my gran in the crowd. It was the most amazing experience.
"It pretty much went to script and, mark my words, that was a good run from Pistorius. It's been a tough road to qualify for the Olympic Games for Oscar. He ran a qualifying time last year but the South African Federation wanted him to repeat the qualifying time this year. Now he has had his chance to represent his country at the Olympics."
"I've got to thank my team, they trust me, I trust them. We've been together for nine years."
He added: "This crowd is amazing. I've run so many times in the UK and it feels like a second home to me."
Pistorius ran in the individual 400m at the 2011 World Championships
but did not progress beyond the semi-finals.
He also ran in the relay, but, after running in the heats, was omitted from the team that won silver in the final.
Pistorius is also due to run in the 4x400m relay at London 2012 before defending his titles at the Paralympic Games.
are also safely through to the semi-finals.
Merritt pulls up in men's 400m