BELGRADE (AFP) - British singer Amy Winehouse, who is struggling to overcome drug addiction, was booed at an open-air concert as she appeared to be too drunk to sing at the performance that officially kicked off a two-week European tour. Some 20,000 people gathered for the highly-promoted concert at Belgrades sixth-century Kalemegdan fortress late Saturday, but many soon left the concert, disappointed with the songstresss lacklustre act. During the concert which lasted about 90 minutes Winehouse could only mumble some of the lyrics, failing to follow her band. She left the stage twice, while many fanssome from neighbouring countriesbooed and whistled as she tried to find a proper note, with her band trying to calm the crowd. I paid my weekly salary, 38 euros ($54) for this concert and I knew of her troubles, but the organisers should have cancelled it to let us, her fans, remember the real Amy, said Bojan Petrovic, a 35-year old teacher. from the southern Serbian town Nis. Winehouse reportedly ended an alcohol rehabilitation programme in London two weeks ago and local media reported that alcohol had been banned for the tour.