(Adds number of detained, injured, details)

By Thomas Grove and Osman Senkul

ISTANBUL, May 1 (Reuters) - Turkish riot police fired water cannon and tear gas in clashes with crowds gathering for an outlawed May Day rally in Istanbul on Thursday, detaining hundreds at a time of heightened political tensions in Turkey.

Thousands of police were stationed across the centre of Turkey's largest city to block access to its main Taksim Square. Three major trade union confederations had pledged to mobilise up to 500,000 people in defiance of an official ban.

Officials banned the rally due to intelligence reports that radical groups planned to stage violent protests. Leftists and Kurdish separatists frequently clash with police at protests.

Strains surrounding the traditional May Day demonstrations of workers' unity were heightened this year by union opposition to a recently passed reform of the social security system which sharply raises the retirement age.

Police drove armoured personnel carriers down the city's main pedestrian street, while security personnel in body armour fired tear gas at masked protestors who had ripped up bricks from the pavement to lob at police.

The clashes came amid growing political tensions triggered by a prosecutor's bid to close the ruling AK Party and ban 71 party officials including Prime Minister Tayyip Erdogan for alleged Islamist activities.

The unions later abandoned plans to march towards the square in the face of the heavy police presence. But fighting went on in the city's sidestreets, and six police were injured, said local broadcaster CNN Turk.

Numerous demonstrators were injured as well, though police did not give details. A Reuters reporter saw several police beat one man with truncheons.

"The workers built the road we are standing on and now we are being crushed on it," said retired Resit Celiktepe, 51, amid the clashes.

May Day demonstrations in Istanbul have been marked by clashes between police and protesters in the past and authorities said they would use force to prevent the rally.

"Everyone must show common sense," state-run Anatolian news agency reported Labour Minister Faruk Celik as saying.

"GOVERNMENT PROVOCATIONS"

"If we can get through May Day today in agreement I believe future May Days will turn into the celebration sought by all workers," he said.

Union confederations said they had abandoned plans to march towards Taksim Square.

"In order not to become a tool of this government's provocations we are sensibly ending our actions here. But we will continue to call the government to account," said Suleyman Celebi, chairman of DISK trade union confederation.

Last year dozens were injured in violent street battles on the 30th anniversary of the deaths of 37 people who were shot by an unknown gunman or trampled to death in May Day demonstrations in Taksim Square in 1977.

May 1, a traditional workers' day holiday across most of Europe, is a normal working day in Turkey, where the government resisted intense union pressure this year to make it a day off. (Writing by Daren Butler and Thomas Grove; Editing by Jon Boyle)

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