MOSCOW (Reuters) - Police in Azerbaijan fired rubber bullets and water cannon at hundreds of opposition protesters demonstrating on Sunday against violence in the military in the oil-rich country.

The protest is one of a series triggered by the sudden death of a soldier, Jeyhun Gubadov, on January 7 at a military barracks.

The Defence Ministry said initially the soldier had died of a heart attack, but his family believed he was murdered and four soldiers were arrested after an investigation was opened.

Some 500 people gathered the capital Baku shouting "No to deaths in the army" at the rally mostly organized via Facebook by young opposition activists. The demonstration was not sanctioned by the authorities.

Western governments and human rights groups accuse Azerbaijan's President Ilham Aliyev, who succeeded his father in 2003, of rigging elections and of clamping down on dissent. Protests are often swiftly broken up by security forces.

Buoyed by oil wealth, Azerbaijan has increased military spending to demonstrate its military power to its neighbor and arch foe Armenia, against which it fought a war in the 1990s.

But experts say the army is still weak and in need of modernization.

(Reporting by Lada Evgrashina; Writing by Alissa de Carbonnel; Editing by Alison Williams)