Thousands of protesters marched tearfully in the Lebanese capital on Thursday, marking two years since a cataclysmic explosion at the Beirut port, with chants denouncing the government's failure to uncover the truth behind the blast.

In a grim reminder of the disaster, several grain silos that were left heavily damaged by the blast collapsed on Thursday afternoon, only hundreds of metres from where crowds were gathering at the city's waterfront.

The concrete silos cracked and fell after burning for several weeks, sending a cloud of smoke and dust into the sky. Protesters covered their mouths in disbelief.

"Seeing the smoke coming out – especially that I was here during the blast – triggers a very bad memory. It was the same smoke coming from the silos up to the sky," said 31-year-old protester Samer al-Khoury.

The protesters, wearing t-shirts stamped with blood-red handprints, were marching from Lebanon's justice ministry to the city's waterfront and then to parliament in the centre of Beirut.

The blast damages swathes of the city on Aug 4, 2020, killing at least 220 people. One of the largest non-nuclear explosions in history, it was caused by massive stores of ammonium nitrate kept at the site in the port and neglected since 2013.

"It's important for me to be here today because it's very important for us to ask for justice and accountability for what happened," said Stephanie Moukheiber, 27, a Lebanese woman living in Canada for the last decade, who decided to spend the summer in Lebanon.

"What happened was not a mistake, it was a massacre. It destroyed an entire city."

Several senior officials have been accused of responsibility but, to date, none have been held to account - symptomatic, critics say, of a governing elite hamstrung by corruption and on whose watch Lebanon has descended into a political and economic crisis.

Lebanon's current President Michel Aoun said days after the blast that he had been warned about the chemical stores at the port and asked security chiefs to do what is necessary.

The prime minister at the time also said he had been informed - but no one warned the population about the dangers of the materials. An investigation into the blast has been stalled for more than six months.

In a mass commemorating the victims on Thursday, Lebanon's top Christian cleric Patriarch Beshara al-Rai said God "condemned" officials stalling the domestic investigation and reiterated calls for an international probe.

"What more do you want, what more than this crime of the century, to act?" said Rai, who retains significant political influence in Lebanon's sectarian power-sharing system.

Shiite Hezbollah and its ally Amal have been the main opponents of the probe, accusing the judge leading it of bias after he charged senior Amal Movement politicians and sought to question other top security and political officials.

The probe has been stalled for months due to the refusal of an Amal-backed minister to sign an administrative decree.

Amal and Hezbollah both issued statements Sunday calling for an end to the politicization of the case and said they support a transparent probe.

Hezbollah leader Hassan Nasrallah on Thursday night said in a televised address that "the solution is for this judge to step down and another judge with credibility to be appointed."

Families of victims have also pressed the U.N. Human Rights Council to establish an international enquiry and on Thursday protested outside the French embassy in Beirut, urging Paris to back an external investigation.

Speaking alongside demonstrators, Human Rights Watch researcher Aya Majzoub said France has blocked efforts to set up an external investigation for political reasons.

In an interview with francophone Lebanese daily L'Orient Le Jour published Wednesday, French President Emmanuel Macron said he had proposed an international investigation to Lebanese authorities, but that they had opted for a local one instead.

"It is a sovereign choice that France and the countries that are friends of Lebanon have respected," he said, adding that the Lebanese probe should resume and take its course.

U.N. Secretary General Antonio Guterres said Thursday marked "two years without justice", and called in a Twitter post for "an impartial, thorough and transparent investigation" - a plea echoed by the European Union's delegation in Beirut.

Pope Francis also said on Wednesday he hoped the people of Lebanon could be comforted by justice over the blast, saying "the truth can never be hidden".

"The scars are still there. Internally, externally," said Omar Jheir, the 42-year-old owner of a coffee shop in Beirut heavily damaged in the explosion.

"Especially (since) we haven't seen justice yet."

(Reporting by Timour Azhari, Maya Gebeily and Emilie Madi; Editing by John Stonestreet, Jane Merriman and Daniel Wallis)