Amnesty International Monday accused the Lebanese authorities of relentlessly and shamelessly obstructing the investigation into last year's deadly Beirut port blast.

A year after the August 4 explosion that killed more than 200 people and leveled entire neighborhoods of the city, no official has been brought to justice.

The blast, considered one of the biggest non-nuclear explosions in history, was caused by hundreds of tonnes of ammonium nitrate stored in a dockside warehouse.

How the fertilizer got there, why it was so poorly stored for years, and what started the fire that blew it up, are questions that the Lebanese probe has yet to answer.

"Lebanese authorities have spent the past year shamelessly obstructing victims' quest for truth and justice," Amnesty International said in a statement.

The explosion was widely seen by the victims' families and the broader public as the result of the incompetence and corruption of the ruling class.

Authorities dismissed the first judge who was tasked with investigating the case, after he summoned top officials for questioning.

His replacement has also faced obstruction from leading political figures, when he also tried to bring in senior politicians and security officials for questioning.

"Given the scale of this tragedy, it is astounding to see how far the Lebanese authorities are prepared to go to shield themselves from scrutiny," Amnesty's deputy regional director Lynn Maalouf said.

The lack of accountability is blocking compensation and insurance payments to the victims, and has further discredited an already reviled political elite.

Relatives of some of the explosion's victims are planning marches on Wednesday to honor their loved ones, on the first anniversary of the tragedy, and to demand that the parliamentary immunity of some of the chief suspects be lifted.