Boxer Claressa Shields says she wants to inspire a generation of downtrodden young Americans across the United States after retaining her middleweight title on Sunday at the Rio Olympics.

The 21-year-old Shields was the first American woman to win boxing gold in London four years ago and on Sunday she successfully defended her crown, attempting acrobatics in the ring afterwards and then standing on the podium proudly displaying both golds.

Shields, who was just 17 when she triumphed in London, paraded around the Rio ring screaming with an American flag fluttering behind her having defeated Nouchka Fontijn of the Netherlands on unanimous points.

Shields has spoken passionately in the past about her tough upbringing in Flint, an economically depressed city in Michigan which suffered badly with the drastic downturn in the American auto industry and has been embroiled in a water-contamination crisis.

"I have a great story to tell and I have been through a lot in my life, but I want to inspire people and help people and give people just a little bit of hope," said Shields, who believes her life is "a little bit bigger than just boxing".

"Because I remember that when I was once of those kids I didn't have any hope and when I got just a little bit, look how far I've been able to come," added Shields.

"That's what I want to do for those kids who grew up in my home city of Flint or grew up in towns anything like it -- Chicago, Memphis -- anywhere they come up somewhere rough or they have bad parents.

"Even if you come from somewhere good and you got bad parents, or you go through bad things in life. Your life is different, you make your decisions and you decide what to do.

"I decided to be great."    

- 'I am great' -                  

Shields, the world champion who has never looked in trouble at any point in Brazil, stalked her unfancied opponent in the first round, swinging wildly but missing on a couple of occasions until she made contact with a heavy right near the end of the round.

The Dutchwoman, 28, had most of the crowd in the 9,000-seat arena on her side  -- it is a case of supporting anyone but an American or an Argentine for many Brazilians in Rio -- and they roared their approval when Fontijn enjoyed some success in the second.

She landed several times on Shields, but the American hardly looked bothered and was starting to turn the screw and came out with real intent at the start of the third round.

The American was building a lead and had her hands down in a sign the confidence was flowing through her, as the pair went at one another at the end of the round with a flurry of wild punches.

Shields only had to avoid trouble in the fourth and final round -- and did that comfortably to retain her title, sparking attempts at a cartwheel and then her ecstatic foray around the ring.

"In 2012 when I won I was excited but I hid a lot of my excitement," she said, explaining her exuberant reaction this time.

"It was one of the happiest days of my life but I just stood on the podium. I had so many thoughts but I should have showed how excited I was.

"This time I wanted to embrace every moment of my second Olympics and that's why I decided to run around the ring with the flag because I am proud of how great I am."