A man holds his child as he casts the ballot at a polling station during a referendum in Tashkent, Uzbekistan, Sunday, April 30, 2023. AP
According to preliminary results released Monday, the changes were approved by 90.21 percent of votes with a 84.54 percent turnout on Sunday.
But election observers from the Organization for Security and Co-operation in Europe (OSCE) said the vote lacked the participation of meaningful opposition and was not truly representative.
Mirziyoyev, 65, became president in 2016 after the death of dictator Islam Karimov.
His proposed reforms, when confirmed, could extend presidential terms from five to seven years, allowing Mirziyoyev to serve two more terms.
"Uzbekistan's constitutional referendum was technically well prepared and widely promoted as a move to enhance various rights and freedoms, but it took place in an environment that fell short of genuine political pluralism and competition," an OSCE statement said.
Despite undertaking recent reforms, government critics and rights groups say Uzbekistan remains authoritarian.
Mirziyoyev has insisted that the overhaul will improve governance and the quality of life.
Neighbouring Kazakhstan was the first to congratulate the Uzbek leader, with President Kassym-Jomart Tokayev saying the result showed "trust" and "firm support from the Uzbek nation."
Following Karimov's death, Mirziyoyev spearheaded a series of reforms, including a clampdown on forced labour in the country's cotton fields.
After years of being cut off from the world under Karimov, Uzbekistan now wants to join the World Trade Organization.
The constitutional changes also include a ban on capital punishment and the protection of human rights for what Mirziyoyev has called a "New Uzbekistan."
But despite some economic progress and social improvements -- such as the criminalisation of domestic violence -- activists say rights abuses persist.
*This story was edited by Ahram Online