Azerbaijan held a referendum today to extend the presidential term from five to seven years, a step that a European democracy watchdog said would hand unprecedented power to President Ilham Aliyev.

Aliyev, 54, succeeded his father as president in 2003 and can seek re-election indefinitely after term limits were scrapped in another referendum seven years ago.

There are 5.3 million eligible voters in the oil-producing, former Soviet republic.

The authorities argue that a longer presidential term will make for more continuity in decision-making, which they describe as vital after a slump in world oil prices halted Azerbaijan's long run of economic growth.

"One of the main tasks of the referendum is implementation of economic reforms in the post-oil boom period in Azerbaijan, the elimination of bureaucratic mechanisms and creation of an effective governing system," said Ali Hasanov, the president's assistant on political issues.

European democracy watchdogs have said previous votes in Azerbaijan have been marred by vote-rigging and unequal access to the media. The Baku government denies the accusations.

The Venice Commission, an advisory unit of the Council of Europe, a rights and monitoring body of which Azerbaijan is a member, said of the referendum: "Many proposed amendments would severely upset the balance of power by giving 'unprecedented' powers to the president."

The referendum has been accompanied by arrests and intimidation from the beginning

Some rights activists also criticised other proposed amendments, which would give the president power to declare early presidential election at his convenience, as well as to dissolve parliament.

"The referendum has been accompanied by arrests and intimidation from the beginning," Denis Krivosheev, Deputy Director for Europe and Central Asia at Amnesty International, said in a statement. "Those who have attempted to criticise and campaign against these proposals have faced assaults and harassment by the authorities."

The Central Election Commission reported a turnout for the referendum of 64 percent by 5 p.m.

"I voted in favour of all changes, although I have not read what those changes are about," Sanuber Muradova, 40, a school teacher, said after casting her ballot. "Politics is not for ordinary people, but I'm a teacher and I have to go and vote."

Aliyev's rule long benefited from an economic boom fuelled by oil pumped to Europe from a region where the West and Russia are vying for influence over huge energy reserves.