The oil-rich Caspian Sea nation of Azerbaijan is set to hold a parliamentary election amid strong criticism from international rights groups, which have accused authorities of limiting free speech.

The leading trans-Atlantic security and rights group, the Organization for Security and Cooperation in Europe, has refused to monitor Sunday's vote after Azerbaijani authorities demanded that it sharply cut the number of observers it was planning to send.

The OSCE said the restriction on the number of observers would make it impossible to monitor the vote effectively. It marks the first time since Azerbaijan won independence after the 1991 Soviet collapse that the OSCE will not monitor its election.

Azerbaijani President Ilham Aliyev's government long has faced criticism in the West for showing little tolerance for dissent and holding elections that fall below democratic standards.

At the same time, Aliyev, in office since succeeding his father in 2003, has firmly allied the Shia Muslim nation with the West, helping secure its energy and security interests and offset Russia's influence in the strategic Caspian region.

The parliamentary race features 767 candidates competing for seats in the 125-member parliament.

Opinion surveys have indicated a strong lead for the ruling Yeni Azerbaijan party, the results reflecting a rise of incomes in the nation of 9 million under Aliyev's watch thanks to the country's oil wealth.

Some leading opposition groups are boycotting the vote. Ali Kerimli, the leader of Azerbaijan's Popular Front, said his party will not take part in the election because "the country lacks a proper political environment and legal base for conducting a democratic vote."

The leading opposition party, Musavat, announced Wednesday that it was pulling out of the balloting.

"We have demanded that the authorities create democratic conditions and equal opportunities for all political forces and parties and put off the vote," said its leader, Arif Hajili.

Hajili said only 24 of the 73 candidates nominated by the opposition party have been registered for it by the authorities.

Election authorities, however, refused to take Musavat off the ballot, saying it's too late to do so, and accused the party of trying to thwart the vote. Some Musavat candidates reportedly have said they would stay on the ballot despite the party leadership's decision.

Amnesty International has protested what it described as "sustained and severe attack" on the freedom of expression in Azerbaijan in the run-up to the parliamentary election after two of its staff were stopped at the border and deported.

Amnesty said there are at least 20 prisoners of conscience in Azerbaijan, who have been convicted on trumped-up charges ranging from fraud to treason.

"Azerbaijani authorities must uphold their human rights obligations and immediately release all prisoners of conscience, as well as stop persecuting civil society activists, including human rights defenders," Denis Krivosheev, Amnesty International's Deputy Director of Europe and Central Asia, said in a statement Friday.

The group said most of the country's independent human rights organizations have been shut down and prominent rights leaders arrested or forced into exile. It said an official crackdown has made activists fearful to speak out against human rights violations, while independent media is now almost non-existent and state-controlled media are used to smear critics.

Azerbaijani authorities have dismissed such criticism as unfounded. Ali Hasanov, Aliyev's aide in charge of domestic politics, has said the vote will be democratic and transparent.

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Vladimir Isachenkov in Moscow contributed to this report.