By Ayla Jean Yackley and Evrim Ergin

ISTANBUL (Reuters) - Turkish police fired tear gas, water cannon and rubber pellets on Thursday to stop May Day protesters, some armed with fire bombs, from defying Prime Minister Tayyip Erdogan and reaching Istanbul's central Taksim square.

Citing security fears, authorities shut parts of the city's public transport system, erected steel barricades and deployed thousands of riot police to block access to Taksim, a traditional union rallying point and the focus of weeks of anti-government protests last summer.

Erdogan, who warned last week he would not let labour unions march on Taksim, has cast both last year's street protests and a corruption scandal dogging his government since December as part of a plot to undermine him.

The Istanbul governor's office said it had received advanced information that "illegal terror organisations and their extensions" would resort to violence to stoke unrest.

But the security measures failed to deter thousands of people from trying to march, with pockets of protesters playing cat and mouse with police in tear gas-shrouded side streets.

Demonstrators in surrounding neighbourhoods repeatedly tried to breach police lines blocking the way to Taksim, a normally teeming shopping and tourism district which lay virtually deserted and ringed by security checkpoints.

Some 40 people were hospitalised and around 160 detained, according to the Progressive Lawyers Association.

In the working class Okmeydani district, members of leftist groups threw fire bombs and fireworks at security forces, who responded with rubber pellets. Similar clashes erupted in March at the funeral of teenager Berkin Elvan, who had lain in a coma after being wounded in last year's unrest.

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