* 18 nationalists detained over alleged clash plans
* Far-right leader warns government
* Erdogan to address southeast rally a day after clashes
By Daren Butler
ISTANBUL, June 1 (Reuters) - Turkish police have detained a group of ultra-nationalists on suspicion that they planned to stir violence at an election rally, media reports said on Wednesday, adding to tensions ahead of the June 12 vote.
Far-right Nationalist Movement Party (MHP) leader Devlet Bahceli responded angrily to the detentions, lashing out at Prime Minister Tayyip Erdogan whose ruling AK Party is set to win a comfortable third consecutive election victory.
Opinion polls on Wednesday showed the AK on course to win around 50 percent support in the election. [ID:nLDE7500D0]
The campaign has already marked by violence at rallies, a spate of militant attacks linked to Kurdish separatists, and a sex video tape scandal that has wounded the MHP, the country's third largest party.
"An extraordinary effort is being exerted to create an atmosphere of chaos and turn the MHP and political separatists against each other," Bahceli said in a written statement.
"The prime minister and government will take prime responsibility for our fellow campaigners who have now been detained and for negative developments after this," he said.
Police detained 18 ultra-nationalists in Istanbul and Izmir after wire-tapping alleged phone conversations indicating they planned to trigger clashes at an MHP rally in Diyarbakir on June 6, the Sabah daily said.
Police confiscated 37 petrol bombs ahead of an Erdogan rally on Wednesday in Diyarbakir, the main city in the Kurdish southeast of Turkey, state-run Anatolian news agency reported.
A day earlier, riot police fired water cannon and tear gas as they battled stone-throwing protesters in the Black Sea town of Hopa, where Erdogan was campaigning.
One of the prime minister's bodyguards was injured as he fell from Erdogan's campaign bus after apparently being hit by a stone. Another man died from an apparent heart attack, media reports said.
Last month, Kurdish militants ambushed an AK campaign bus in the northern province of Kastamonu, killing one policeman.
The MHP, which has little support in the southeast, is highly critical of government efforts to solve the Kurdish problem which it sees as a threat to national unity.
Erdogan launched an initiative in 2009 to grant Kurds greater cultural rights, including the establishment of a Kurdish-language television channel and teaching of Kurdish at university, but Kurdish parties want political reform and autonomy.
The MHP has been rocked by a scandal over secretly filmed videos apparently showing party leaders engaged in extra-marital affairs and making compromising political statements. Ten party leaders have resigned as a result.
Opinion polls suggest the scandal has not harmed support for the MHP which stands around 10-13 percent, above the 10 percent threshold which parties must exceed to enter parliament. (Writing by Daren Butler; Editing by Simon Cameron-Moore and Paul Taylor)