A UN-backed tribunal has found Hezbollah member Salim Jamil Ayyash guilty in the 2005 assassination of former Lebanese prime minister Rafik al-Hariri, with the three remaining defendants acquitted due to a lack of evidence.

The judge said Ayyash was one of the main actors behind the assassination.

He was found guilty of planning and carrying out the terrorist attack, as well as the murder of Hariri and 21 other people, and now faces a life sentence.

Sentencing is scheduled for September.

The guilt of the three other men, Hassan Habib Merhi, Assad Hassan Sabra and Hussein Hassan Oneissi, could “not be proven beyond doubt,” the judge ruled, noting that the evidence was almost entirely circumstantial.

Hariri was killed in a attack that hit his motorcade in Beirut on February 14, 2005.

Alongside the politician, 21 other people were killed and 226 injured by the massive suicide truck bombing caused by 3000 kilograms of explosives that generated a shock wave that could be felt for kilometres.

His son Saad Hariri speaks to the media after the tribunal’s ruling. Photo: AAP

Hezbollah has denied involvement in the attack and the judges in the Netherlands said no evidence implicated the militant group’s leadership.

One of the judges called the assassination “undoubtedly a political act,” noting that Hariri had wanted to stand for re-election and end Syrian dominance in Lebanon.

His son, Saad, said he accepted the verdict and called on his followers not to take to the streets after the verdict to avoid tensions with supporters of the Hezbollah movement.

“Today, the sacrifice must come from Hezbollah: It has become clear that the network of killers were from their ranks, and they believe that for this reason they will not be brought to justice,” said Saad Hariri, who is also a former prime minister.

“I will not rest until they are handed over to justice and punished,” he said.

Dozens of Hariri’s followers gathered to pray around his grave in central Beirut shortly after the verdict was delivered.

Hariri’s sister, Bahia, as well as some of his relatives and followers, were seen reading verses from the Koran and leaving white flowers at his grave.

Some of his followers were in tears.

UN chief Antonio Guterres called on the international community to respect the decision of a UN-backed tribunal.

“The judgement in the case is a reflection of the international community’s commitment to justice for the terrible crimes committed on that day,” Guterres said.

The Special Tribunal for Lebanon (STL) was established in 2007 and has been trying the accused in absentia for the past six years.

It has now made legal history with the first terrorism trial by an international court.

The tribunal issued an indictment and arrest warrants for the four men in 2011.

None of the accused were in custody and the trial against them, which started in 2014, has been held in absentia.