Equatorial Guinea's main opposition have dismissed as a sham the referendum on limiting presidents of the oil-rich nation to serving two terms.
Pablo Mba Nsang of the Convergence for Social Democracy Party told the BBC it was a way for the president to stay in power for another 14 years.
The government says that, with more than three-fifths of votes counted, 99% of voters have backed the referendum.
President Teodoro Obiang Nguema Mbasogo has been in power for 32 years.
It is unclear whether President Obiang, Africa's longest-serving president, will have to step down when his current term ends in 2016, the AFP news agency reports.
Critics say the changes to the constitution, which include the creation of the post of vice-president, will allow him to hand-pick his successor.
"There clearly is a worry that his eldest son Teodoro is being groomed to become the next president of the country," Joseph Kraus, who works for the Washington-based rights group Equatorial Guinea Justice, told the BBC's World Today programme.
Teodoro Nguema Obiang Mangue, who currently serves as the country's agriculture minister, has been accused of using his ministerial position and his father's influence to plunder his nation's wealth.
Last month, the US government said it was seeking to recover assets worth more $70m (£44m) from him.
The Convergence for Social Democracy Party withdrew its agents from polling stations on Sunday, complaining of irregularities.
Mr Nsang said there was evidence of ballot-stuffing during the referendum.
"It's not a secret ballot," he told the BBC's Network Africa programme.
"People were voting for absentees. People will vote for their [dead] relatives. People will vote many times."
Information Minister Jeronimo Osa Osa Ecoro told AFP the vote had passed off peacefully.
President Obiang, 69, seized power from his uncle and has so far served four seven-year terms.
Since the mid 1990s - when multi-party politics was allowed - the former Spanish colony has become one of sub-Saharan Africa's biggest oil producers.
But few people have benefited from the oil riches and the UN says that less than half the population have access to clean drinking water while 20% of children die before reaching five.