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PRISTINA (Reuters) - Thousands of Kosovo opposition supporters marched through Pristina on Tuesday blaming the government for the death of an imprisoned activist suspected of being involved in a grenade attack on the parliament building.

Astrit Dehari, 26, and five other members of the hardline nationalist Vetevendosje political party were arrested in September on suspicion of being behind the rocket propelled grenade attack.

The attack came when the parliament was due vote on a border deal with neighboring Montenegro, a prerequisite for closer ties with the European Union including visa-free travel. Vetevendosje, the largest opposition party, opposes the deal.

Dehari was found dead in his prison cell on Saturday and authorities are investigating. His party suspects he was murdered and blames the government for not providing sufficient security in jail.

“The regime in Kosovo has the direct responsibility for the death of Astrit,” Vetevendosje’s leader Visar Ymeri said.

Tension has been rising over the past year among political parties. Opposition MPs have released tear gas inside parliament several times to protest against an accord with Serbia giving more autonomy to Serb-held areas of Kosovo and the border deal with Montenegro.

Opposition supporters clashed with the police several times during the same period.

Opposition parties reject the border agreement, which is yet to be passed by the parliament, saying it hands over some 8,000 hectares of Kosovo territory to Montenegro.

Kosovo declared independence from Serbia in 2008 and is still not recognized by Belgrade. It has signed a trade and political pact with the EU but five member states - Spain, Cyprus, Romania, Greece and Slovakia - still do not recognize it.