Hundreds of protesters gathered in Lebanon on Friday to mark the third anniversary of a devastating explosion that ripped through a port in Beirut, demanding accountability from officials over a disaster that remains shrouded in mystery.

Footage from Lebanese media showed demonstrators taking to the streets during a nationwide three-day mourning period and chanting slogans against politicians they accused of obstructing the investigation into the blast, which killed at least 200 people and injured 6,000 on August 4, 2020.

The incident at the Port of Beirut in the country's capital was one of the world's largest non-nuclear explosions.

The blast sent up a huge mushroom cloud-shaped shockwave, flipping cars and leveling buildings.

It registered as a 3.3 magnitude earthquake and was felt hundreds of miles away, as far as Cyprus.

Investigators attributed the blast to approximately 2,750 tons of seized ammonium nitrate that had been stored in a port warehouse since 2014.

But three years on and what caused the ammonium nitrate to ignite remains a mystery.

An investigation aimed at prosecuting several top politicians for criminal neglect has come to a standstill, with activists and legal experts urging the United Nations to initiate a fact-finding mission to uncover the truth.

Earlier this week, caretaker Prime Minister Najib Mikati declared the anniversary of the blast a nationwide holiday.

He also launched a nationwide three-day mourning period for those affected by the blast.

"Public institutions and municipalities should close on Friday, August 4, 2023 in memory of the tragedy of the port explosion as a show of solidarity with the families of the innocent martyrs and the injured and their families," he said.

Meanwhile, French President Emmanuel Macron has pledged support for the former French colony.

In a tweet Friday, Macron recalled his visit to Lebanon days after the explosion, saying he was "at the side" of the Lebanese people.

"Lebanon was not alone. It still isn't. You can count on France, our solidarity, our friendship," the French leader said.

Last month, Macron appointed his former foreign minister, Jean Yves Le Drian, to the role of special envoy to Lebanon as part of France's effort to end the political deadlock in the country.