Several marches are set to converge on the port in the afternoon to remember the victims and demand justice.
Nobody has been held responsible for the August 4, 2020 disaster -- one of history's biggest non-nuclear explosions -- which also injured at least 6,500 people and devastated swathes of the capital.
Authorities said the explosion was triggered by a fire in a warehouse where a stockpile of ammonium nitrate fertiliser had been haphazardly stored for years.
An investigation has stalled, mired in legal and political wrangling.
"The complete lack of accountability for such a manmade disaster is staggering," United Nations Special Coordinator for Lebanon Jeanine Hennis-Plasschaert said in a statement on Saturday.
"One would expect the concerned authorities to work tirelessly to lift all barriers... but the opposite is happening," she said, calling for "an impartial, thorough, and transparent investigation to deliver truth, justice, and accountability".
In December 2020, lead investigator Fadi Sawan charged former prime minister Hassan Diab and three ex-ministers with negligence, but as political pressure mounted, he was removed from the case.
His successor, Tarek Bitar, unsuccessfully asked lawmakers to lift parliamentary immunity for MPs who were formerly cabinet ministers.
In December 2021, Bitar suspended his probe after a barrage of lawsuits, while the powerful Hezbollah group has accused him of bias and demanded his dismissal.
But in January last year, he resumed investigations, charging eight new suspects including high-level security officials and Lebanon's top prosecutor, who in turn charged Bitar with "usurping power" and ordered the release of detainees in the case.
The process has since stalled again.
A judicial official, speaking on condition of anonymity, told AFP that Bitar would "resume his proceedings, starting next week" and intends to finish "the investigation and issue his indictment decision... by the end of the year".
Bitar will set dates for questioning defendants who have not yet appeared before him, according to the official.
If the public prosecutor's office or other relevant judicial officials fail to cooperate, Bitar "will issue arrest warrants in absentia" for the defendants, the official added.
Activists have called for a UN fact-finding mission into the blast, but Lebanese officials have repeatedly rejected the demand.
Prospects of further disaster loom over this year's anniversary, with Hamas ally Hezbollah and the Israeli army trading cross-border fire since the Palestinian group's October 7 attack that triggered the Gaza war and fears that an all-out conflict could engulf Lebanon.
