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BELGRADE, May 5 (Reuters) - Serbian President Boris Tadic has received death threats for "betraying the Serb people" by seeking closer ties with the European Union despite its support for Kosovo's secession, officials said on Monday.

Tadic, also the leader of the main pro-Western party, backed the signing of a Stabilisation and Association Agreement last week over the objections of nationalists, who said it amounted to a recognition of independence for Serbia's former province.

"We are aware of the death threats President Tadic received," the state prosecutor's office said on Monday. "Relevant state institutions are taking all measures necessary to protect the president and identify the perpetrators."

The president's office declined to comment on the threats. The Serbian daily Blic quoted one letter as saying Tadic would get a bullet to the head for betraying the country.

The secession of Kosovo, Serbia's medieval heartland, polarised Serbian society between hardliners who want to freeze ties with the EU and pro-Western liberals who think there is a way to move towards membership and still not give up Kosovo.

The government, a coalition of Tadic's Democrats with outgoing nationalist Prime Minister Vojislav Kostunica, collapsed under the strain. Campaigning for the May 11 election has focused largely on the EU-Kosovo dilemma.

Tadic, speaking to reporters at a public function, urged politicians to calm down and not create 'bad blood in the country' for the sake of a few more percentage points in the election.

There were several mafia-style murders during Slobodan Milosevic's autocratic rule in the 1990s. The most recent and traumatic was the 2003 assassination of reformist Prime Minister Zoran Djindjic, shot by a sniper connected to nationalist crime groups.

Police are also investigating an alleged plot against war crimes prosecutor Vladimir Vukcevic, who has angered nationalists by pursuing suspects from the Yugoslav wars of the 1990s. (Reporting by Ivana Sekularac; Editing by Ellie Tzortzi and Tim Pearce)

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