(Reuters) - Lady Gaga and Tony Bennett, bridging a 60-year age gap to form one of music's unlikeliest pairs, launched a jazz album on Monday that lets the pop diva put aside her wacky image to sing sweet harmonies with the elder statesman of cool.

"Cheek to Cheek," on sale on Tuesday after a launch concert in the ornate Renaissance setting of the Grand-Place in Brussels, features jazz standards by Cole Porter, Duke Ellington and others a distant world away from Lady Gaga's 21st century.

"When I began writing music for the music industry, I became known as the quirky girl from downtown New York," she told a news conference at the 15th-century city hall.

"So I tailored my music to be that way, to get noticed, to be able to travel more and play more shows," said the 28-year-old artist known to many as much for startling costumes - like a robe made of meat - as for her innovative music and stage shows.

But swathed in blue velvet with a 2-metre (6-foot) train, Lady Gaga this week was more 1950s Hollywood star than queen of MTV in 2014 - although still easily young enough to be Bennett's grand-daughter.

"I feel liberated," she said, holding the hand of her fellow Italian-American New Yorker and musing on the challenges of old-style jazz. "It's been over eight years and I've not been singing out. But Tony will not accept any less than all of me."

The two first met in 2011 at a charity concert in New York and a short time afterwards, recorded a version of "The Lady is a Tramp" for Bennett's album Duets II.

"I was overjoyed that he had heard that I'd been singing jazz for so long, perhaps I was even afraid that I had lost that part of me," Lady Gaga said.

Bennett, who shot to fame with "Because of You" in 1951, said he enjoyed the simplicity of a jazz setting.

"I started the very same way as she did, with thousands of people cheering," he said.

"Later on I said I want to keep it in a simple way. Instead of the big stadiums, I'd like to play in fine acoustical homes."

For the concert to launch the album, the two chose a grand but intimate outdoor location, setting up stage on the Grand-Place, a site of pomp and pageantry down the centuries.

Once again, Lady Gaga kept it simple, sticking to a glittery gold cocktail dress and refraining from any of the costume changes that her live performances are known for.

Another change was the composition of the approximately 5,000-strong audience on the square, combining Lady Gaga's young fan base with many middle-aged spectators.

For about 30 minutes, the two performed some of the highlights of the album, from the upbeat duet "It Don't Mean a Thing" to Lady Gaga's solemn solo performance "Lush Life."

The crowd in Brussels responded enthusiastically to the brief show and Lady Gaga appeared touched by the reaction as she and Bennett bowed out.

Before the concert, Bennett had already predicted that Lady Gaga would strike a chord with fans as a jazz singer.

"I know that when Lady Gaga comes out you will hear a reaction that's never been heard in this great city," Bennett said. "People love her, and she loves them back."