Bruce Jenner: "Caitlyn doesn't have any secrets."

AFP/Los Angeles

Caitlyn Jenner, the transgender Olympic champion formerly known as Bruce, unveiled her new name and look on Monday in a Vanity Fair covershoot - drawing widespread praise and smashing a Twitter record.

Transgender campaigners - and many well-wishers - welcomed the high-profile debut, as did the 65-year-old Jenner's family, which includes the media-savvy celebrity Kardashian clan.

Jenner managed to break President Barack Obama's record for amassing one million Twitter followers in the shortest time - about four hours.

"I'm so happy after such a long struggle to be living my true self," Jenner wrote in her first tweet after the magazine released the July cover photo by celebrity photographer Annie Leibovitz.

"Welcome to the world Caitlyn. Can't wait for you to get to know her/me," added Jenner, who sported a cream-coloured strapless bodysuit and long brown hair for the July issue of Vanity Fair.

In a video clip of the photo shoot on the magazine's website, Jenner said: "Bruce always had to tell a lie. He was always living that lie. Every day, he always had a secret, from morning till night.

"Caitlyn doesn't have any secrets," said Jenner, a reality television star, who is also shown in a black evening gown in the video.

"As soon as the Vanity Fair cover comes out, I am free," she said.

"Caitlyn Jenner" and the hashtag #CallMeCaitlyn quickly rocketed to the top of the Twitterverse as the top trending topic in the US and worldwide.

Jenner broke Obama's Twitter record for the shortest time to reach one million followers, achieving the mark with her @Caitlyn_Jenner account in four hours and three minutes.

Obama - who joined Twitter as @POTUS only last month - took about 4.5 hours to reach the one million mark.

"Another Jenner world record, and at 65. Who'da thought! Humbled and honored to have reached 1M followers in 4 hours. Thank you for your support," she said in only her third tweet.

Outpouring of support

Jenner was widely praised for revealing in a television interview broadcast in April that she was transitioning to life as a woman, earning praise for coming out as transgender.

Jenner's family quickly rallied around her on Twitter.

"How beautiful! Be happy, be proud, live life YOUR way!" wrote Kim Kardashian, Jenner's step-daughter and fellow reality television star.

The outpouring of support also included a message from @BarackObama, the Twitter handle run by the remnants of Obama's campaign staff, which the president has sometimes tweeted from by signing off with a "-bo."

"It takes courage to share your story," it said, retweeting one of Jenner's posts, although Obama did not sign the message.

Jenner became an American sports hero by setting a world record in the decathlon in the 1976 Olympics.

But it was through a previous marriage to Kris Kardashian that Jenner once again became a household name with the reality TV series Keeping Up with the Kardashians, appearing as the sometimes bemused stepfather of her flamboyant daughters.

The couple married in 1991 and have two daughters together, Kendall and Kylie. They recently divorced.