They have also condemned Aliyev s surprise - and unexplained - decision to hold the election six months ahead of schedule, saying it was aimed at shortening the campaign period and hampering efforts to prevent vote-rigging.
At a polling station in the capital Baku, 38-year-old schoolteacher Elmira Balayeva said she had voted for Aliyev because he was the only candidate capable of steering the country towards economic wellbeing and political stability.
The 56-year-old Aliyev has ruled the South Caucasus country of almost 10 million people since shortly before his father's death in 2003.
Turnout was 39.4 percent at 0800 GMT, four hours after polls opened, the head of the Central Election Commission said.
"We have never seen free elections in this country". Opinion surveys have put support for the incumbent at over 80 percent (Washington Post) and opposition parties say they are boycotting the elections because of Aliyev's record.
Aliyev, 56, has led Azerbaijan since 2003.
The April 11 presidential election is the first election after the Constitutional amendments of the country, which enabled Aliyev to run for office for the 4th time for a 7-year term.
The changes drew criticism from Council of Europe constitutional law experts as "severely upsetting the balance of powers" and giving the president "unprecedented" authority. The ramps will also facilitate the persons using wheelchair to come to polling stations independently and vote.
The former Soviet republic's huge energy reserves and its strategic location along the Caspian Sea mean it is viewed by Europe as an important alternative to Russian Federation for energy supplies.
In February 2017, Aliyev appointed his wife, Mehriban Aliyeva, as first vice president - a post also created by the referendum - placing her first in line to take over if the president dies or is incapacitated.
Opposition leaders say these "dummy candidates" were hand-picked by the authorities to try to make the vote look competitive.
But critics argue they have crushed the opposition and used their power to amass a fortune that funds a lavish lifestyle for the president and his family.
At the same time, his government has long faced criticism in the West for alleged human rights abuses and suppression of dissent.
Moreover, the CEC has accredited 890 global and 58,175 domestic observers to monitor the presidential election.The worldwide observers represents 59 countries and 60 organisations including the OSCE Parliamentary Assembly, the OSCE Office for Democratic Institutions and Human Rights (ODIHR), PACE, CIS who are coming to Azerbaijan to monitor the presidential election.The basic objective is to ensure transparency.
The country has about 5.3 million people out of a total population of 9.9 million registered voters.
The Central Election Commission was to begin announcing official results later Wednesday.