By Nick Mulvenney

RIO DE JANEIRO (Reuters) - Bahamian Shaunae Miller launched herself over the finishing line with a desperate dive to pip American Allyson Felix to Olympic 400 meters gold by the slimmest of margins in a thrilling final on Monday.

After literally flying across the line in a personal best time of 49.44 seconds, the 22-year-old lay on the track, shaken up and exhausted, as she waited for the scoreboard to confirm she had clinched her first major title.

"I didn't see anyone until the last 20 meters and the only thing I was thinking was that I must get that gold medal," said Miller.

"I think (the dive) was just a reaction, my mind went blank, the next thing I knew I was lying on the ground. I've got a few cuts and bruises but I'm okay.

"This is the moment I have been waiting for. I am just so happy, so grateful, such emotions I just can't say."

Felix, who finished in 49.51, was also on the floor after failing to claim her fifth Olympic gold medal, let alone the 200-400 double she had originally planned for the Rio Games.

"Disappointment," said the 30-year-old, struggling to hold back her tears. "It's been a tough one and I really hoped it would come together tonight.

"I just gave it all I had, I don't think I had anything left to give. I feel emotionally and physically drained at this moment. I really wanted it, it's painful."

Felix was denied the chance to defend her 200m title, and bid for an unprecedented double, after failing to finish in the top three at the U.S trials while suffering the effects of an ankle injury she suffered in a gym accident earlier this year.

The Californian's silver medal was, though, her seventh in four Games, the most of any American female track and field athlete at the Olympics.

Silver medalist in the 200m in Athens in 2004 and again in Beijing four years later, Felix won her only individual gold over the half-lap in London in 2012. She has also won three relay golds and could add to that tally in Brazil.

Second behind Felix in the 400 meters at the world championships in Beijing last year when her strong finish was not enough to rein in the American, Miller had clearly learned her lesson.

She exploded out of the blocks in lane seven and went for broke, her long stride giving her a good lead on the back straight as the shorter Felix powered away inside her.

Miller retained the lead coming off the final bend but started tightening up as Felix surged about 50 meters from the line and the pair were neck-and-neck until the Bahamian's gold medal-winning dive.

Jamaica's Shericka Jackson took bronze in 49.85 ahead of Americans Natasha Hastings (50.34) and Phyllis Francis (50.41) in fourth and fifth.

(Editing by Ed Osmond/Sudipto Ganguly)