LONDON (AP) -- The video is painful to watch, an artistic train wreck for all to see. The question is what Amy Winehouse is going to do next.

The five-time Grammy Award winner, whose battles with alcohol and drug abuse have long overshadowed her music career, had a very public meltdown on stage Saturday night in Belgrade, the first stop of her European summer concert tour.

The disastrous performance shattered the goal of the troubled singer's comeback tour: A renewed focus on her musical ability, not her substance abuse problems. It also raised serious doubts about whether she is well enough to perform before an audience.

Winehouse was jeered and booed as she stumbled around the stage unable to remember the lyrics to her own songs. At times she could barely stand up. She was not able to carry a tune, appearing disoriented and unaware of her surroundings. At one point she dragged over a backup singer to take the mike and sing.

The crowd at Belgrade's Kalemegdan Park was often unable to decipher even what song Winehouse was performing -- a startling problem for a singer who is popular in Serbia. Many of the 20,000 fans walked out in protest, angry that they paid steep ticket prices for such a spectacle.

Serbian media called the concert a "scandal," with the Blic daily labeling it "the worst in the history of Belgrade."

Instead of continuing to Istanbul on Monday and Athens on Wednesday for long-scheduled concerts, Winehouse canceled those concerts and returned to her London home. She is now holding talks that are expected to lead to further cancellations of her 12-date European tour.

The Belgrade meltdown has raised questions about Winehouse's future viability as a live act.

Winehouse, 27, has been hospitalized twice for injuries suffered after fainting and falling at home, and her father said she has health problems stemming from smoking cigarettes and crack cocaine.

Neil Warnock, chief executive of The Agency Group booking agency, said Winehouse and her tour promoters may be entitled to insurance coverage for any cancellations. He said insurance settlements would depend on what previous information was provided to the insurance companies.

"If you fairly disclosed any pre-existing conditions, and what caused the cancellation is a new condition, then the artists and promoters would be covered," he said. "If it's a pre-condition, then it wouldn't be covered, or if it's an undeclared condition that should have been declared, that wouldn't be covered."

He said the legal and financial issues that followed Michael Jackson's sudden death in advance of a series of London performances were based on similar questions.

Warnock said even bands with long histories of substance abuse like the Rolling Stones were traditionally able to meet their contractual obligations and perform at their gigs. He would not speculate directly on Winehouse's future.

The British press reported that Winehouse had turned down payment for the Belgrade fiasco, and Serbian fans flooded the Internet with demands for refunds. Her management did not comment on the reports.

The singer's next scheduled concert is July 8 in Bilbao, Spain, but it is likely to be canceled. The tour was to end in Bucharest, Romania, on Aug. 15. The demanding schedule was supposed to be an important test of Winehouse's ability to stand up to the rigors of live performing.

In addition to her stage troubles, her recording career has also been put on hold. There have long been plans for a follow up to her two successful albums -- including the breakthrough "Back to Black" released in 2006 -- but new material has not been released.

It has been a dramatic fall for Winehouse, whose grasp of pop and soul -- along with her trademark beehive hairdo and her raunchy stage act -- brought her worldwide fame and substantial sales.

Her first album "Frank," released in 2003, was heavily influenced by contemporary jazz and earned her critical acclaim. "Back to Black" arrived three years later and was an overwhelming success with its unusual fusion of jazz, pop and soul with a heavy debt to the girl groups of the early, pre-Beatles 1960s.

It was edgy as well, with the song "Rehab" dealing with the health issues that were soon to sidetrack her musical career.

"They tried to make me go to rehab," Winehouse sang on the hit. "I said 'No, no, no."'

Winehouse has sought rehab therapy in the past after her widely publicized battles with alcohol and drugs.