Mazahir Panahov told journalists that with more than half of the votes counted, just over 92% of the electorate had backed a proposal to get rid of a two-term presidential limit. Turnout was around 71%, or 3,478,571 voters.
"The percentages in favor are so high that we can already say that all the changes will be approved," he said.
He also said that a wide range of other constitutional amendments proposed in the referendum would be approved. These include media restrictions, and a law against showing "disrespect" to state symbols.
Opposition leaders have criticized the referendum, saying it was designed to allow President Ilham Aliyev, 47, to remain in power for life. Aliyev won a second five-year term last October in elections that were criticized by OSCE monitors.
Aliyev's late father, Heydar, was the leader of Soviet Azerbaijani, and also ruled the country for just over a decade following the break-up of the U.S.S.R. The Aliyev family has been in power in one form or another in Azerbaijan for four decades. Opponents of the referendum had called for a boycott, hoping that a turnout of below 25% would see the vote deemed invalid.