Turkey's main Kurdish political party has appealed for international support after hundreds of its officials were arrested in a crackdown by Turkish authorities.

The Democratic Society party (DTP) has written to members of the European parliament asking them to speak out against the arrests, which follow the party's surprise success in last month's local elections. The DTP, the fourth largest party in the Turkish parliament with 20 seats, fears that the arrests will radicalise the Kurdish minority and make a solution to the Kurdish problem even more elusive.

About 40,000 people have died in the 25-year conflict between the Turkish authorities and the outlawed Kurdistan Workers' party (PKK). The DTP insists that the campaign for Kurdish language and cultural rights be pursued through political means, but the Turkish military considers the party a PKK front.

The DTP almost doubled the number of municipalities under its control from 56 to 98 in last month's elections and came first in 10 provinces in eastern and south-eastern Turkey.

The results were a blow to Turkey's prime minister, Recep Tayyip Erdogan, and his Justice and Development party (AKP). The party won almost 39% of the vote, eight points less than in a general election two years ago, and lost 15 mayoralties. Erdogan had described the election as a referendum on his leadership and said that anything less than 47% of the vote would be a failure.

The poll setback came despite a strong drive against the DTP in its strongholds in the south-east. Allegations were made of unfair practices, including handing out washing machines and other gifts to voters to persuade them not to vote for the DTP. Unable to defeat the DTP at the ballot box, the AKP was now resorting to rougher measures, analysts said.

"Before the election, the AKP were talking about having good relations with Kurdish regional governments, an economic development plan and some cultural reforms," said Mesut Yegen of the Middle East Technical University in Ankara. "But they wanted to do so from a position of strength; they do not recognise the PKK and the DTP as legitimate actors. Erdogan has not grasped the seriousness of the Kurdish question."

A Turkish court last week sentenced the mayor of Diyarbakir, Osman Baydemir, and the mayor of Batman, Nejdet Atalay, to 10 months in jail for spreading PKK propaganda. In condemning a Turkish military incursion against PKK bases in neighbouring Iraq in February last year, Baydemir had said that "neither soldiers nor guerrillas should die". For using the word "guerrillas" he was charged with "spreading PKK propaganda" and "inciting separatism" under Turkey's strict laws on freedom of speech.

The DTP is also facing the threat of being shut down in a case before the constitutional court. Analysts say it is hard to see how the latest moves against the DTP will not influence the case, even though the evidence has already been compiled.

Ahmet Turk, the president of the DTP, struck a defiant note at a talk at Chatham House in London last week. He told journalists: "They may put me in prison, they may kill me, but the struggle for Kurdish rights will continue."

Human rights groups have expressed concern at the targeting of the DTP. "The secrecy order on the investigation prevents us from knowing what the precise evidence consists of, but this is not a very constructive approach to the issue of minority rights in Turkey, an area that has seen very little progress in its negotiations on EU membership," said Emma Sinclair-Webb, a researcher for Human Rights Watch in Istanbul.

Britain, a strong backer of Turkey's EU membership bid, said the arrests were a matter for the Turkish courts, but added that it supported pluralism.