ISTANBUL (Reuters) - Turkish police fired water cannon and tear gas to prevent hundreds of protesters from defying a ban on May Day rallies and reaching Istanbul's central Taksim Square on Thursday.

Citing security fears, authorities shut down parts of the city's public transport system and deployed thousands of riot police, blocking access to Taksim, a traditional union rallying point and the epicentre of weeks of protests last summer.

Flag-waving demonstrators attempting to breach police lines were met with water cannon and tear gas in the Besiktas neighbourhood on the edge of the Bosphorus not far from Taksim, a Reuters witness said.