Niyazov had been in power in his reclusive Central Asian state, the second largest natural gas producer in the former Soviet Union, since 1985.

There was no designated successor and Niyazov's death raises concerns about the transfer of power in the country, with foreign oil and gas companies sizing up opportunities for investment.

Niyazov's health had been a top secret in Turkmenistan where he went by the title Turkmenbashi (Head of the Turkmen) the Great.

"Turkmenbashi the Great has died," said a news presenter on state television.

The presenter said the 66-year-old leader had died of cardiac arrest. A government source said the Turkmen leader died overnight.

The television ran still images of a national flag in a black-bordered frame.

The government was due to hold an emergency meeting later in the day, the television said.

A special commission has been set up to work out the details of Niyazov's funeral.

Deputy Prime Minister Kurbanguly Berdymukhamedov was named to head the commission. In Soviet times, this person often became the successor, but it was not clear if this precedent would be followed in Turkmenistan.

Opposition sources said Berdymukhamedov was a relative of Niyazov, though this has never been acknowledged officially.

Niyazov tolerated no dissent and enjoyed a flourishing personality cult with thousands of portraits and statues of him throughout the country.

His name has been given to a sea port, farms, military units and even to a meteorite.

A Reuters correspondent in the Turkmen capital said the situation was calm but workers were seen removing New Year decorations from fir trees in the streets. Reuters