Turkmenistan's authoritarian president Saparmurat Niyazov, who ruled the Central Asian country for 21 years, has died aged 66, state TV has reported.
State TV said Nizayov's death was a "great loss" to the Turkmens
Mr Niyazov, who named cities and airports after himself in a personality cult, left no designated successor.
Turkmenistan, which has large gas reserves, now faces an uncertain future with rival groups and outside powers scrambling for influence, analysts say.
Mr Niyazov died at 0110 local time (2010 GMT Wednesday) of a heart attack.
Last month, the president publicly acknowledged he had heart disease.
His funeral is set to take place on 24 December in the capital, Ashgabat.
BBC correspondents quote witnesses as saying the capital has been quiet since the news broke, with many people staying at home, shocked and unsure of what may happen next.
Deputy Prime Minister Gurbanguly Berdymukhamedov has been named head of the commission handling the funeral, state television said.
Became head of Communist Party in Turkmenistan in 1985
Styled himself Turkmenbashi, Father of the Turkmen
Built up a cult of personality, with cities, airports and months named after him
Introduced bizarre laws, banning young men from wearing beards and listening to car radios
Poverty remained rife in the energy-rich country
According to Turkmen law, the president is succeeded by the head of the legislative body, the People's Assembly. But this post was held by Mr Niyazov himself.
Turkmenistan has called an emergency meeting of its highest representative body for 26 December to decide on Mr Niyazov's succession, the government said.
Mr Berdymukhamedov has also been named acting head of state until then, according to government sources.
The cabinet of ministers and the National Security Council in Turkmenistan have held emergency sessions to discuss the situation.
Russian Foreign Minister Sergei Lavrov has called for power in Turkmenistan to be transferred "in the framework of the law" to ensure stability in the region.
"We hope that a new leadership will act to benefit co-operation with Russia and to benefit the region as a whole," he said.
The president of neighbouring Afghanistan, Hamid Karzai, said he was saddened at the news of Mr Niyazov's "unfortunate" death.
Mr Karzai said Mr Niyazov "played a key role in strengthening bilateral relations" between the two countries.
Many in the region fear it is less the bizarre style of his rule and more the lack of political institutions that could prove to be the real legacy of Mr Niyazov, says the BBC's Natalia Antelava in Almaty, Kazakhstan.
Education, healthcare and society generally are regarded as having crumbled under his rule.
"President Niyazov was in effect the state and what he decreed on any subject, from politics, to culture to science, was absolute law," says Michael Hall, Central Asia project director for the International Crisis Group.
A mostly Muslim nation, Turkmenistan boasts the world's fifth largest natural gas reserves as well as substantial oil resources.
Cult of personality
Mr Niyazov became Communist Party chief of what was then a Soviet republic in 1985 and was elected first president of independent Turkmenistan in 1991.
In 1999, he was made president-for-life by the country's rubber-stamp parliament.
During his reign, Mr Niyazov established a cult of personality in which he was styled as Turkmenbashi, or Leader of all Turkmens.
He renamed months and days in the calendar after himself and his family, and ordered statues of himself to be erected throughout the desert nation.
Cities, an airport and a meteorite were given his name.
Mr Niyazov was intolerant of criticism and allowed no political opposition or free media in the nation of five million people.
His laws became increasingly personal. It was forbidden to listen to car radios or smoke in public, or for young men to wear beards.
An alleged assassination attempt in 2002 was used to crush his few remaining opponents.
All candidates in the December 2004 parliamentary elections, at which there were no foreign observers, were his supporters.