* Serbian youths expelled on visit to historic site
* Health authorities in Serbia say some had gunshot wounds
* Police say expelled Serbs were aggressive and drunk (Adds more detail on injured, NATO reaction)
By Fatos Bytyci and Aleksandar Vasovic
PRISTINA/BELGRADE, June 28 (Reuters) - More than 50 people were injured in clashes on Thursday when the authorities in Kosovo deported a group of visiting Serbs who accused the police of shooting at them, leaving one with life-threatening gunshot wounds.
The group of about 70 mostly young Serbs was travelling in two buses to Gazimestan, a religious and historic site close to the capital Pristina, when police turned them back, arguing they had become "very aggressive, drunk and were provoking both police and citizens".
Kosovo declared its independence from Serbia in 2008, but tensions between the 90 percent Albanian majority and the small Serb minority have persisted, and relations with Belgrade itself have remained strained.
Serbian health authorities said one Serb sustained life-threatening injuries in the clashes and that five others were hospitalised with gunshot wounds. A total of twenty Serbs sought medical care in the towns of Kursumlija and Prokuplje, just outside Kosovo.
Police in Kosovo declined to confirm whether they had fired live rounds at the Serb group.
Doctors in Gracanica, a Serb municipality close to the capital Pristina, said they had treated 17 injured people after Molotov cocktails and stones were thrown at the Serbs' buses.
NATO's mission in Kosovo also said Molotov cocktails had been thrown at the convoy - as it passed through Pristina - and condemned the violence.
"This shameful action is contrary to the integrity of the maintenance of a safe and secure environment for all people in Kosovo," it said in a statement.
In Pristina, Kosovo President Atifete Jahjaga said the authorities would stop all such groups in future "as they are seriously violating law and order and further exacerbating the security situation in Kosovo".
Kosovo's Interior Minister Bajram Rexhepi said the Serb youths had thrown stones and other heavy objects at the police shortly after being expelled from Kosovo territory.
Nine Kosovo police officers were treated in hospital and a further 26 p olicemen suffered minor injuries, he added.
Gazimestan is a field just outside Pristina, where Serbs mark St. Vitus Day and the 1389 Battle of Kosovo, when an Orthodox Christian Serb force led by Tsar Lazar lost a decisive battle to invading Muslim Ottoman Turks.
Ivica Dacic, Serbia's prime minister-designate, said the incident was damaging for peace and stability in Kosovo.
"International troops there have an obligation to preserve peace and security ... All future talks (with Kosovo) must be based on the preservation of security," Dacic told reporters in Belgrade.
In Gazimestan, where the ceremony took place, police searched buses and seized nationalist paraphernalia from visiting Serbs.
Gazimestan is also the place where on June 28 1989 late Serbian strongman Slobodan Milosevic addressed hundreds of thousands of Serbs in a speech that heralded the bloody collapse of communist six-republic Yugoslavia in the 1990s.
On Thursday, Serbs holding photographs of Milosevic and of other former Serb politicians and generals facing war crimes charges at the International Criminal Tribunal for the former Yugoslavia in The Hague shouted: "We want weapons, we want weapons."
Violence between Albanians, Serbs and international peacekeepers flared last year, after Pristina attempted to establish its authority in the northern, predominantly Serb part of the country which still pledges allegiance to Belgrade.
Independent Kosovo has been recognised by more than 90 countries, including the United States and 22 of the European Union's 27 member states. However, Russia and China as well as Serbia itself have refused to recognise it.
In a separate incident on Thursday, police in Kosovo arrested three local Serbs for throwing stones at a local bus.
Violence also flared briefly at Kosovo's parliament after a fist fight involving two MPs from the ruling party and two others from the outspoken opposition party Vetevendosje (Self-determination) broke out. (Additional reporting by Branislav Krstic in Mitrovica; Editing by Zoran Radosavljevic and Andrew Osborn)