African leaders joined Ethiopian Prime Minister Abiy Ahmed in officially unveiling the Grand Renaissance Dam (GERD) that has promised to revolutionise the country's energy sector but also caused tensions with neighbouring Egypt.

The dam is the largest by power capacity on the continent and could transform a country where almost half the 130-million population still lacks electricity, according to World Bank data.

"This country that was dark in the evening when I first arrived here... is now selling energy to neighbouring countries," Pietro Salini, CEO of Webuild, the main contractor for the project, told AFP.

Kenya, Tanzania and South Sudan have already agreed deals to buy the electricity.

The dam towers 170 metres (550 feet) high and stretches nearly two kilometres (1.2 miles) across the Blue Nile near the Sudanese border.

The $4-billion megastructure is designed to hold 74 billion cubic metres of water and generate 5,150 megawatts of electricity -- more than double Ethiopia's current capacity.

More than 25,000 people, mostly from Europe and Ethiopia, laboured on the giant site from 2011 to its official opening.

Salini said his firm spent some 250,000 hours training workers in health and safety and technical engineering, noting: "The human factor is always the hardest to tackle."

Neighbouring countries Egypt and Sudan are wrong to worry about the impact of the dam on their water supply, he said.

"The hydroelectric project releases water to produce energy. They are not irrigation schemes that consume water," said Salini.

"There's no change in the flow. It's just regulated," he added.

Salini also said the project was entirely financed by Ethiopia.

"Not one international lender was willing to put money in this project," he told AFP.

A brutal civil war between the government and rebels from the Tigray region between 2020 and 2022, which claimed roughly 600,000 lives, also slowed the GERD'S completion.

But all those challenges are now in the past, Salini said.

"At an opening ceremony, you don't think about past difficulties," he added.