ANKARA (REUTERS) - Germany's president warned during a trip to Turkey on Monday against the consequences of curbing freedom of expression and he criticised Prime Minister Tayyip Erdogan's leadership style.

A heavy-handed crackdown on anti-government demonstrations since last year and a corruption scandal swirling around Erdogan's inner circle have generated the greatest challenge to the premier's 11-year rule and drawn the outspoken disapproval of senior officials in the European Union.

Gauck, a former East German Lutheran pastor who campaigned for the 1989 fall of the Berlin Wall, praised Turkey for the progress made under Erdogan on issues such as the economy and the Kurdish peace process, but added: "Recently we have also heard voices of disappointment, bitterness and outrage at a style of leadership which many see as a risk to democracy," he said in an address at the Middle East Technical University in Ankara.

Gauck said he was shocked by the government's attempts to stamp out street protests and clamp down on the media - a move that saw Twitter blocked for two weeks in the run-up to recent local elections, won comfortably by Erdogan's AK Party.

He spoke as security forces geared up for possible clashes with demonstrators during May Day marches planned for Thursday. "All my life, experience tells me that where freedom of expression is curtailed, where people are not fully informed, not consulted nor allowed to participate, resentment and harshness, and finally also the willingness to resort to violence, grow," Gauck said.

"It should be praised as a sign of political maturity that Turks are prepared to act as citizens who want to actively shape the future of their country... Countless people of Turkish origin who live in Germany share this view. They want to see democratic discussions in the country of their ancestors."

Turkish security forces wielding pellet guns, water cannon and tear gas have repeatedly clashed with protesters since last May, when the country was rocked by huge demonstrations against what critics seee as Erdogan's growing authoritarianism. Nine people, including a policeman, have been killed in the unrest.

Police in Istanbul were preparing barricades in Taksim Square - the epicentre of last year's protests - on Monday after trade unions promised to march there as part of May Day celebrations, despite being refused official permission.

Gauck, whose position is largely ceremonial but exerts moral authority, also took aim at recent Turkish legal changes and a purge of the police and judiciary which saw thousands of officials removed in an effort by the government to limit corruption investigations.

He questioned whether the judiciary could preserve its independence under such conditions.