Turkish police used water canon and tear gas on Thursday to disperse hundreds of protesters who tried to defy a Labour Day ban on demonstrations on Istanbul's Taksim Square, the scene of protests that have dogged the government for months.   After giving a final warning, hundreds of riot police backed up by water cannon trucks moved in on protesters in the Besiktas district as they tried to breach the barricades leading up to the symbolic square, according to an AFP reporter.