A UN-backed tribunal has found a member of the Hezbollah Shia movement guilty over the 2005 murder of former Lebanese prime minister Rafik Hariri, but cleared three other suspects after a years-long trial.

The long-awaited decision prompted mixed reactions, with the late Hariri's son Saad telling journalists outside the Special Tribunal for Lebanon he "accepted the tribunal's verdict".

"Everybody's expectation was much higher than what came out today, but I believe that the tribunal came out with a result that is satisfying. We accept it," Mr Hariri said after attending the hearing at the heavily fortified courthouse.

Salim Ayyash, 56, was convicted in absentia over the huge suicide truck bombing in Beirut that killed the Sunni billionaire politician and 21 other people.

Rafik Hariri was killed in 2005

"The trial chamber finds Mr Ayyash guilty beyond reasonable doubt as a co-perpetrator of the assassination of Rafic Hariri," said David Re, presiding judge of the tribunal.

Addressing victims of the attack, he said: "We sincerely hope the verdict today will give you some sort of closure."

But judges said there was not enough evidence to convict Assad Sabra, 43, Hussein Oneissi, 46, and Hassan Habib Merhi, 54, over the blast, which changed the face of the Middle East.

The judges also said there was no evidence to directly link Syria, the former military overlord in Lebanon, or Hezbollah's leadership to the attack.

Sentencing for Ayyash will be decided at a later date. He faces life imprisonment if he is ever brought before the court.

Hezbollah chief Hassan Nasrallah has refused to hand over the four defendants and rejected the court's legitimacy.

The assassination of Lebanon's Hariri and its aftermath

Mobile phone plot

Judges said there was sufficient evidence to show that Ayyash was at the centre of a network of mobile phone users who scoped out Hariri's movements for months before his assassination.

A still-unidentified suicide bomber driving an explosives-laden Mitsubishi truck blew himself up as Hariri's motorcade passed on the Beirut waterfront on Valentine's Day in 2005.

Prosecutors had said Ayyash was a ringleader of the group, while Mr Oneissi and Mr Sabra allegedly sent a fake video to the Al-Jazeera news channel claiming responsibility on behalf of a made-up group.

Merhi was accused of general involvement in the plot.

The judges said evidence also linked phones used in the attack to Hezbollah commander Mustafa Badreddine, who was indicted by the court but is believed to have been killed in the Damascus area in May 2016.

The bombing triggered mass protests that drove Syrian forces out of Lebanon after three decades.

But the court said there was not enough evidence to tie Syria to the crime.

"Syria and Hezbollah may have had motives to eliminate Mr Hariri and his political allies, however there is no evidence that the Hezbollah leadership had any involvement in Mr Hariri's murder and there is no direct evidence of Syrian involvement," Judge Re said.

The hearing opened with a minute's silence for victims of the explosion that devastated Beirut two weeks ago, killing 177 people.

The verdicts were initially scheduled for 7 August but postponed because of the blast.

Judge Re called on the court to observe a "minute's silence to remember the victims of this catastrophe, those who lost their lives, those who were maimed or injured, their families, those who were made homeless".

Saad Hariri speaks to the media after the tribunal's verdict

The UN Security Council agreed in 2007 to establish the court billed as the world's first international tribunal set up to probe terrorist crimes.

It opened its doors in 2009, although the Hariri trial itself did not formally start until 2014.

The court has cost at least €500m to operate and has so far heard only four cases, two of them for contempt of court about news reports with information about confidential witnesses.

Prosecutors said during the trial that Hariri was assassinated because he was perceived to be a "severe threat" to Syrian control of the country, allied to Saudi Arabia and the United States.

Hariri was Lebanon's Sunni premier until his resignation in 2004 over Syria's role as powerbroker in the country.

Observers have voiced fears that the verdict, whichever way it goes, could spark violence on the streets in Lebanon.

But Lebanese President Michel Aoun said afterwards that "achieving justice resonates with everyone's desire to uncover the circumstances behind this heinous crime that threatened stability and civil peace in Lebanon".

The verdict came as thousands of Beirut residents have expressed anger at the authorities after the port blast triggered by a warehouse fire that set off large amounts of stored ammonium nitrate.

The disaster led to the Lebanese government's resignation and compounded Lebanon's severe economic crisis.