Aliyev's son, current Prime Minister Ilham Aliyev, is among eight candidates competing to succeed the 80-year-old president, who remains in the United States at a Cleveland hospital.
Two opposition leaders are seen as the younger Aliyev's main challengers: Isa Gambar of the Equality Party, and Etibar Mamedov of the National Independence Party.
The election has already been criticized by international groups, including the Organization for Security and Cooperation in Europe, which cited incidents of police violence at opposition campaign rallies, intimidation, and biased media coverage, Radio Free Europe reported.
The OSCE has sent some 600 monitors to observe the election.
The New York-based Human Rights Watch also said the legitimacy of the election will be questionable no matter how the actual voting is conducted.
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