Even at 50, the queen of pop just can't stop courting controversy.

As Madonna kicked off her international "Sticky and Sweet" tour Saturday night, she took a none-too subtle swipe at the presumptive Republican nominee for U. S. president.

Amid a four-act show at Cardiff's packed Millennium Stadium, a video interlude carried images of destruction, global warming, Nazi dictator Adolf Hitler, Zimbabwe's authoritarian President Robert Mugabe - and U. S. Senator John McCain. Another sequence, shown later, pictured slain Beatle John Lennon, followed by climate activist Al Gore, Mahatma Gandhi and finally McCain's Democratic rival Barack Obama.

U.S. singer Madonna performs on the opening night of her "Sticky & Sweet" tour at the Millennium Stadium in Cardiff August 23, 2008. Madonna kicks off her "Sticky & Sweet" world tour at Cardiff's Millennium Stadium on Saturday, the latest test of her enduring appeal just a week after her 50th birthday.

The rest of the show had the usual Madonna fixtures: sequins, fishnets and bondage-style outfits drawn from the 3,500 items of clothing reported whipped together by 36 designers specifically for the tour. Dancers sauntered across stage in top hats and tail coats and Madonna tried her hand at break-dancing and pole-dancing.

Some 40,000 fans - many in pink cowboy hats and boas - were treated to a heavy-metal version of "Borderline," while "La Isla Bonita" served as backdrop for a flamenco routine. The show, billed as a musical mishmash of "gangsta pimp," Romanian folk, rave and dance - was an homage to Madonna's reinventions over the last three decades.

She did a playful take on her variegated career, mocking dancers dressed as her previous incarnations - including the "Material Girl" and "Blonde Ambition" - before they sank into the stage to the tune of "She's Not Me." Madonna finished off the concert with her thumping "Give it 2 Me" from her new urban-inspired album, "Hard Candy."

If the world's top-selling female recording artist is still writhing, shaking and shimmying with the best of them, her personal life has recently been unsettled. Earlier this summer her brother Christopher Ciccone published a gossipy memoir and she has faced speculation about her relationship with New York Yankee slugger Alex Rodriquez and rumours her marriage to British filmmaker Guy Ritchie is on the rocks - which she hotly denies.

Madonna's tour was eagerly anticipated in Britain, where the pop superstar has made her home and fans weren't disappointed.

"We enjoyed it to the max," said Ruth Henson, 24, of London.

"Madonna, considering she's now 50, is so fit. She did a really good job."

Following Cardiff's opening concert, "Sticky and Sweet" moves across Europe, hitting London's Wembley Stadium on Sept. 11 and Paris on Sept. 20. From there, it goes to North America in October before wrapping up Dec. 18 in Sao Paulo, Brazil.

It is Madonna's first tour since striking a deal with concert promoter Live Nation Inc. worth an estimated $120 million over 10 years. The partnership gives Live Nation a stake of future music and music-related business she generates, including touring, merchandising and albums. Madonna's last tour was her 2006 "Confessions" - in which she staged a mock crucifixion only a few kilometres from the Vatican.