Azerbaijani President Ilham Aliyev on Wednesday sealed an overwhelming election victory to secure a third term and further extend his family's decades-long rule, in a vote rejected as illegitimate by the sidelined opposition.

Aliyev scored a crushing win in the tightly-controlled ex-Soviet state with almost 85 percent of the vote, with main opposition challenger Jamil Hasanli in distant second place, partial results showed.

"The presidential election in Azerbaijan was a triumph for democracy," Aliyev said in a televised address to the nation early Thursday morning.

"The fact that this vote was free and transparent is another important step towards democracy," Aliyev said.

But Hasanli's election campaign has already alleged there were "massive" electoral violations across the resource-rich country and promised not to accept the result.

Aliyev had an unassailable lead in the elections with some 84.7 percent of the vote, the central election commission said, in partial results based on around 75 percent vote count. Hasanli was in second on just over 5.2 percent of the vote.

The 51-year-old Aliyev came to power in a disputed 2003 vote after the death of his powerful father Heydar, a former KGB officer and Communist-era boss who ruled the ex-Soviet nation of 9.5 million people for the preceding 10 years.

He was re-elected in 2008 with 89 percent of the vote in a poll called neither free nor fair by Western observers and pushed through a referendum a year later that allowed him to run this time round.

Aliyev, who has a vice-like grip over most media in the country, has stayed away from televised debates and passed up public rallies ahead of the polls.

An initial exit poll from the officially-accredited Prognos agency put Aliyev on almost 84 percent of the vote, far ahead of Hasanli, in second place with around eight percent.

Some 72 percent of Azerbaijan's roughly five million registered voters cast their ballots, the central electoral commission said.

'Massive violations across the country'

Main challenger Hasanli, however, has rubbished the election, alleging a string of violations, including voters being bussed round to cast ballots at multiple polling stations, vote-stuffing and observers being barred from monitoring the vote.

"Massive violations were carried out across the country," the Hasanli campaign said in a statement.

The Hasanli camp "does not accept the election result or recognise it as free or fair," the statement said.

Fuelled by billions of petrodollars, living standards in the mainly Muslim nation have soared in the past decade, with Azerbaijan becoming an increasingly important energy supplier to Europe and an ally of NATO.

During voting at one polling station in Baku -- a city which has undergone a glitzy building boom in recent years -- voters waited in silence to cast their ballots under an Azerbaijani flag.

"Under his (Aliyev's) leadership the country has really developed," said Rizvan Samedov, a 25-year-old marketing director.

But not all said their country was on the right track.

"I have lost faith in the current leadership," said Samaya Alekperova, a teacher, adding that she would vote for the united opposition candidate.

Normally fragmented, Azerbaijan's weakened opposition -- much of which boycotted the 2008 poll -- in May seemed primed for a genuine challenge after rallying around a single candidate, but have faced major obstacles getting their message heard.

Hasanli, who has attracted thousands of supporters to rallies, has pledged to step down after two years if elected and switch Azerbaijan to a parliamentary system.

Rights groups accuse the authorities of an intense clampdown on dissent ahead of the election, including the jailing of scores of critics on charges the opposition say are trumped-up.

"It's hard to keep up with the sheer number and the speed at which dissenters are being persecuted at the moment," said John Dalhuisen, Amnesty International's Europe and Central Asia Director.

Aliyev is expected to continue treading a cautious path between the West and Russia after his expected victory, ensuring that Azerbaijan remains a key energy source for Europe and a US ally while not upsetting its giant northern neighbour Russia.