Four Hezbollah members are on trial in absentia at a UN-backed tribunal accused of murdering former Lebanese prime minister Rafiq Hariri in a 2005 car bombing.

The trial opened in The Hague nine years after the huge Beirut blast that killed the billionaire politician.

Today's proceedings began just hours after a car bomb killed at least three people in a Hezbollah stronghold near the Lebanese border with war-ravaged Syria.

"We will proceed as if the accused are present in the courtroom and have pleaded not guilty," judge David Re told the Special Tribunal for Lebanon (STL).

"The onus is on the prosecutor to prove their guilt."

The STL is unique in international justice as it was set up to try the perpetrators of a terrorist attack and because it can try the suspects in absentia.

A packed public gallery looked on as the repeatedly-delayed proceedings began, with a large scale model of Beirut where the 2005 attack happened on a table before judges.

Mr Hariri's son Saad sat at the back of the courtroom behind the victims' representative. His hands were folded as he listened attentively.

The blast on 14 February 2005 killed 22 people and wounded 226. It led to the establishment by the UN Security Council of the STL in 2007.

Although the attack was initially blamed on pro-Syrian Lebanese generals, the court in 2011 issued arrest warrants against Mustafa Badreddine, 52, Salim Ayyash, 50, Hussein Oneissi, 39, and Assad Sabra, 37, all members of Hezbollah.

A fifth suspect, Hassan Habib Merhi, 48, was indicted last year and his case may yet be joined to the current trial.

"No one in Lebanon can fail to have been affected directly, or indirectly, by the attack in downtown Beirut that on 14 February 2005, killed Mr Rafiq Hariri," said chief prosecutor Norman Farrell.

"The people of Lebanon have a right to this trial and to seek the truth," he said, showing the court a photograph taken shortly after the blast of smoke, flames and Mr Hariri's vehicle on fire.

The four suspects have been charged with nine counts, ranging from conspiracy to commit a terrorist act to homicide and attempted homicide.

Prosecutors allege that Mr Badreddine and Mr Ayyash "kept Mr Hariri under surveillance" before the Valentine's Day suicide bombing, while Mr Oneissi and Mr Sabra allegedly issued a false claim of responsibility to mislead investigators.

Mr Hariri, Lebanon's Sunni prime minister until his resignation in October 2004, was on his way home for lunch when a suicide bomber detonated a van full of explosives as his armoured convoy passed.

A video was then delivered to the Beirut office of Al-Jazeera in which a man "falsely claimed to be a suicide bomber on behalf of a fictional fundamentalist group called 'Victory and Jihad in Greater Syria'," prosecutors said.

They will aim to prove the four men's involvement through tracking their alleged use of mobile phones before, during and after the attack.

Vincent Courcelle-Labrousse, Mr Oneissi's court-appointed lawyer, said before the trial that "there is a huge disproportion between the prosecution and the defence's means, time and financial resources".

"We must defend the accused, who are not even here and without having had any contact with them."