With lines of empty red chairs, one for each of the 11,541 victims of the siege of Sarajevo, Bosnia on Friday remembered when war broke out 20 years ago and the West dithered in the face of the worst atrocities in Europe since World War Two.

The anniversary finds the Balkan country deeply divided, power shared uneasily between Serbs, Croats and Muslims in an unwieldy state ruled by ethnic quotas, and languishing behind its ex-Yugoslav neighbours on the long road to the European Union.

Underscoring the disunity, Bosnia's autonomous Serb Republic ignored Friday's solemn remembrance of the day shots fired on peace protesters in downtown Sarajevo marked the start of the 1992-95 war.

Between April 1992 and December 1995, some 100,000 people died and almost half the country's 4.4 million people were forced to flee their homes; all on NATO's doorstep, a few hours' drive from Vienna or across the Adriatic from Italy.

The siege of Sarajevo by Serb forces that held the hilltops lasted 43 months. Queuing for water or shopping at the market, Sarajevans were picked off by snipers and random shelling. More than 600 children were among them.

In a blood-red symbol of loss, empty chairs stretched 800 metres down the central Sarajevo street named after socialist Yugoslavia's creator and 35-year ruler, Josip Broz Tito. Tributes will include a choir performance of 750 Sarajevo students.

We were moving targets with only one principle left -- that we would stay in the city, Bosnian artist Suada Kapic said of the siege.

On Thursday, cellist Vedran Smailovic, who became an icon of artistic defiance when he played on a central Sarajevo street as the city was shelled, played again for the first time in his hometown since he left in 1993.

I think the majority of the people in this country realises that all of us came out of this war as losers, but I fear the majority has also failed to learn the lessons, said 46-year-old Radoslav Zivkovic, a Serb in the wartime Serb stronghold of Pale.

Bosnia was Tito's Yugoslavia in a bottle, a mix of mainly Orthodox Serbs, Catholic Croats and Muslim Bosniaks.

But as Yugoslavia began to fall apart, and Bosnia's Muslims and Croats voted in a referendum in favour of independence, Serb forces with the big guns of the Yugoslav army seized 70 percent of Bosnian territory, driving out non-Serbs in a policy known as ethnic cleansing.

DISCRIMINATION

The Muslims and Croats fought back, and for a time against each other.

The United Nations sent blue-helmeted peacekeepers but gave them no mandate to shoot back. It was only after the so-called U.N. safe haven in Srebrenica fell in July 1995 to Serb forces, who then massacred 8,000 Muslim men and boys, did NATO use force, eventually bombing the Serbs to the negotiating table.

In the wake of the end of the Cold War, Bosnia taught the world tough lessons in humanitarian intervention that could yet be echoed in Syria, where the United Nations is probing the possible deployment of unarmed monitors under a peace plan to end the conflict between rebels and forces under President Bashar al-Assad.

The Bosnian war ended with a peace deal in 1995 brokered by the United States at a U.S. airbase in Dayton, Ohio.

It silenced the guns but split Bosnia into two ethnically-based regions, power distributed along ethnic lines in a complex state that has stifled development. Ethnic bickering left Bosnia without a central government for the whole of 2011.

The Serb republic's president, Milorad Dodik, reiterated on Friday that the country's collapse was a matter of time.

The West can only delay it, but it will never stop it, he said in an interview with the Serb Banja Luka daily Nezavisne Novine. They have to understand that Serbs want the Serb Republic, and not Bosnia.

Bosnia's neighbour Croatia will join the EU in July next year. Serbia, which under strongman Slobodan Milosevic conspired with Croatia to dismember Bosnia, became an official candidate for EU accession last month, in part as reward for capturing Bosnian Serb wartime commander and genocide suspect Ratko Mladic.

Bosnia is yet to apply. To do so, it must first amend its Dayton-era constitution to reflect a ruling by the European Court of Human Rights that its strict system of ethnic quotas is discriminatory.

The Serbs reject any move towards greater centralisation, regarding Bosnia as an artificial construct foisted upon them by outside powers.

Less than half of the 2 million driven from their homes have returned to the towns and villages they left. Once a vibrant blend of Serbs, Croats and Bosniaks, Sarajevo is now overwhelmingly Bosniak.

(Additional reporting and writing by Matt Robinson; Editing by Angus MacSwan)