DIYARBAKIR, Turkey (Reuters) - Turkey detained the leader of a pro-Kurdish party and the mayor of the southeastern city of Van on Thursday while taking control of three municipalities in the region, the party said, pressing on with a crackdown on Kurdish politicians.

Police fired water cannon and tear gas to disperse a crowd of around 100 people who gathered outside the Van municipality building to protest against the detention of Mayor Bekir Kaya, video footage obtained by Reuters showed.

Turkey is fighting an insurgency by Kurdistan Workers Party (PKK) militants in the largely Kurdish southeast, but the arrest of pro-Kurdish politicians and journalists, part of a wider security clampdown in the wake of a failed July coup, has raised fears among Western allies for human rights in the country.

The co-leader of Kaya's Democratic Regions' Party (DBP), Kamuran Yuksek, was detained by police in Sirnak province, near the Syrian border, the party wrote on its Twitter account. It did not say why he was held.

It also said the government had appointed administrators to run the council in Van and in nearby Siirt and Mardin provinces.

Video showed plainclothes police leading Kaya away from the council offices in the city on the shores of Lake Van, the heart of a province with a population of 1.1 million.

The DBP pledged to resist what it described as the "taking hostage" of its representatives and the seizure of 34 of its municipalities by Turkey's ruling AK Party.

"Using the coup as an excuse, the AKP and government have declared war on all opposition circles in the country, chiefly the Kurdish people ... Our people's answer to these occupation attacks will be to resist," it said.

Police searched municipality offices and the houses of Kaya and four other council officials, the state-run Anadolu agency said. Kaya was sentenced to 15 years in jail in January on a charge of "terror group membership", the agency said, but was appealing against the verdict.

JUDGES, PROSECUTORS DISMISSED

The crackdown on pro-Kurdish politicians has run parallel with a purge of people accused of ties to U.S.-based cleric Fethullah Gulen, blamed by Turkish authorities for masterminding July's coup attempt. Gulen denies the accusation.

More than 110,000 people have been sacked or suspended in the military, civil service, judiciary and elsewhere, while 36,000 people have been jailed pending trial as part of the investigation into the failed putsch.

In the latest clear-out, judicial authorities dismissed 203 judges and prosecutors over links to what Ankara terms the "Gulenist Terror Group", Turkey's Official Gazette, which publishes state rulings, said on Thursday.

Human rights groups and some Western allies have voiced concern at the scope of the purges, fearing President Tayyip Erdogan is using the coup as a pretext to curtail dissent.

Thousands of Kurdish politicians have been detained including dozens of mayors and the leaders of the Peoples' Democratic Party (HDP), parliament's second biggest opposition party, accused of links to the PKK.

The PKK, listed as a terrorist group by Turkey, the United States and European Union, launched an insurgency against the state in 1984 in which more than 40,000 people have been killed. The HDP and other Kurdish parties deny direct ties and say they are working for a peaceful resolution of the conflict.

(Writing by Daren Butler; Editing by Nick Tattersall/Mark Heinrich)