BLOOD supplies are down to emergency levels and hospitals have been forced to cancel non-essential surgery after stocks ran low over the Easter weekend. Chief executive of the Hong Kong Red Cross Blood Transfusion Centre Dr Susan Leong Su-tsing described the shortage as ''acute'' and urged people, especially those with common A and O types, to give blood. ''We have enough blood for emergencies but not for elective surgery, although the shortage was anticipated and measures have already been taken to replenish stocks so scheduled operations can be re-started,'' she said. To help boost blood supplies, the transfusion centre's blood bus is being brought back into service at the Hong Kong Island Star Ferry today after a two-month absence. The service's second donor bus will be stationed on the Yuen Long Estate, Mondays to Fridays, from tomorrow until May 14.