* Police seal off municipal headquarters in Diyarbakir
* Opposition says authorities strike blow against democracy
* Dozens of mayors dismissed in 2016 on similar charges (Adds reaction from political parties)
By Umit Ozdal
DIYARBAKIR, Turkey, Aug 19 (Reuters) - Turkey on Monday replaced mayors from a pro-Kurdish party with state officials in three cities and detained more than 400 people for suspected militant links, the Interior Ministry said, a move likely to fuel tensions in the mainly Kurdish southeast.
President Tayyip Erdogan had warned ahead of nationwide local elections in March of such a move against elected officials if they were found to have had connections to the outlawed militant Kurdistan Workers Party (PKK).
The mayors in Diyarbakir, Mardin and Van, major cities in the mainly Kurdish southeast, were accused of various crimes including membership of a terrorist organisation and spreading terrorist group propaganda, the ministry statement said.
"For the health of the investigations, they have been temporarily removed from their posts as a precaution," it said, referring to Diyarbakir Mayor Selcuk Mizrakli, Mardin Mayor Ahmet Turk and Van Mayor Bedia Ozgokce Ertan.
CNN Turk showed police sealing off the municipality headquarters in Diyarbakir with metal barriers, as riot police deployed outside along with water cannon vehicles.
The Interior Ministry said the PKK had been seeking to exploit the March elections to boost its support and had exerted efforts to nominate candidates who would be easily led once elected.
Police detained 418 people in 29 provinces in a related investigation targeting suspects with links to the outlawed PKK group, the ministry added.
The pro-Kurdish Peoples' Democratic Party (HDP), to which the three mayors belong, said they had been dismissed "on an order based on lies and illegal justifications".
"This is a new and clear political coup. It is a clear and hostile stance against the political will of the Kurdish people," the HDP executive board said in a written statement.
It said the three mayors had been elected with between 53% to 63% of the vote in their cities in March and called for support from other political parties.
"This is not just the problem of the HDP and the Kurdish people. It is the shared problem of all Turkey's peoples and all democratic forces," it added.
MAIN OPPOSITION SLAMS DISMISSALS
Veli Agbaba, deputy leader of the main opposition Republican People's Party (CHP), wrote on Twitter that the dismissals were tantamount to fascism and a blow against democracy, while Istanbul's CHP mayor Ekrem Imamoglu also slammed the move.
"Negating the will of the people is unacceptable," he wrote on Twitter. Imamoglu himself was removed from office over irregularities shortly after coming to power in the March election, but won a re-run election in June.
The removal of the mayors echoed the dismissal of dozens of mayors in 2016 over similar accusations, part of a purge that began after a failed coup. Nearly 100 mayors and thousands of party members were jailed in a crackdown that drew expressions of concern from the United States and European Union.
Ahead of the March election Interior Minister Suleyman Soylu had said 178 candidates were being investigated over alleged PKK links.
Erdogan at the time warned that HDP mayors could again be dismissed if they, like their predecessors, were deemed to have ties to militants.
Erdogan frequently accuses the HDP of links to the PKK, which is designated a terrorist group by Turkey, the EU and the United States. The HDP denies such links.
The PKK launched an insurgency against the Turkish state in 1984. More than 40,000 people have been killed in the conflict.
The Interior Ministry said recent operations had led to PKK militant numbers falling to their lowest level in 30 years, with the number of fighters in Turkey falling to some 600 from around 1,800-2,000 in the past. (Writing by Daren Butler; Editing by Dominic Evans and Gareth Jones)
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