SADNESS hung over a packed house during the last of a series of Andy Lau concerts at the Hongkong Coliseum last night, following news that a 17-year-old youth had died after being injured at an earlier performance. Lee Kar-cheong collapsed and fell as people scrambled for balloons during a show on February 13 and died of severe concussion on Sunday. There was no special ceremony last night to commemorate his death, although Lau broke down during his penultimate song, A Smile In Despair. Fighting back tears, Lau said: ''As an artiste, all I wanted was to entertain the public and make people happy. I deeply regret that this tragedy has happened.'' Lau sang his last song, Goodbye Now, and bowed out amid applause. The joint organisers of the concerts - Yiu Wing Entertainment Company, Engineering Impact Ltd and Teamwork Production House - cancelled their planned end-of-series celebrations last night. Senior Inspector Phyllis Kwan Kam-bing said investigators would take statements from 20 witnesses. She said a report had already been submitted to Mr Justice Bohkary, who is expected to submit to the Governor today his final report on the Lan Kwai Fong New Year stampede, which left 21 dead. Ms Kwan said that since the accident six police officers had been deployed to watch crowds entering and leaving the Coliseum. World Vision of Hongkong, which will organise a ''Famine 30'' charity concert with Commercial Radio at the Coliseum next month, has promised to try to stop patrons from leaving their seats. World Vision's senior officer Miss Alice Kwong Lai-kit said the expected 3,000 participants in the 30-hour charity fast would be advised to ''remain in their seats throughout the event''. ''Commercial Radio was fully in charge of organising the concert and stage details last year. This year we will discuss the programme with them to improve safety for the audience,'' Miss Kwong said. Meanwhile, the Urban Services Department, which manages the Coliseum, will review its crowd control measures and criteria for approving concerts. A spokesman said the request to release 1,500 balloons from the ceiling at the beginning of Lau's concert was given approval after detailed examination. ''We could not see a problem with it because balloons do not have sharp edges and are not heavy,'' the spokesman said. ''We will maintain the policy of scrutinising organisers' programme suggestions thoroughly and review the deployment of security guards, particularly during special effects arrangements.''