BAKU, Azerbaijan — A sweeping majority of voters in Azerbaijan have supported extending the presidential term and powers in the ex-Soviet nation, according to preliminary results released Tuesday.

Azerbaijan’s Central Election Commission chief Mazahir Panahov said that with virtually all ballots counted from Monday’s referendum, 84.2 percent of the votes backed a constitutional amendment extending the presidential term from five to seven years.

Some opponents say the changes would cement what they see as effectively a dynastic rule in the oil-rich Caspian Sea nation.

Other proposed amendments, which included granting the president the right to dissolve parliament, creating new vice presidential jobs and canceling age limits, also won overwhelming support from voters.

Turnout was high at nearly 70 percent, according to election officials.

President Ilham 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. At the same time, his government has long faced criticism in the West for alleged human rights abuses and suppression of dissent.

In Tuesday’s statement, the opposition National Council of Democratic Forces denounced the referendum as “illegitimate and not reflecting the people’s will” and called upon international structures to pressure the Azerbaijani government to annul its results.

Last week, the Venice Commission, a watchdog body of constitutional law experts based in France, criticized the constitutional amendments put on the referendum, saying that extending the presidential mandate “cannot be justified” and that other proposed legal changes would upset the balance of power. It specifically warned that a measure giving the president the power to dissolve parliament would weaken political dissent.

The Venice Commission experts are part of the Council of Europe, a rights authority whose 47 members, including Azerbaijan, have signed the European Convention on Human Rights.

In a statement before the vote, Amnesty International said the constitutional amendments would give more authority to the already powerful president and grant the government even more power to interfere with freedom of assembly, in violation of international standards. It added that preparations for the vote were marked by arrests and intimidation of critics of the proposed constitutional amendments.

The Azerbaijani government has rejected the criticism, saying that the constitutional amendments were intended to cut the red tape and speed up economic reforms.