The trial opened in a suburb outside The Hague nine years after the huge Beirut blast that killed the billionaire Hariri and just hours after another car bombing killed at least three in a Hezbollah stronghold in Lebanon near the border with war-ravaged Syria.
The Special Tribunal for Lebanon (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.
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 equivalent to 2.5 tonnes of TNT as his armoured convoy passed.
The February 14, 2005 seafront blast killed 22 people including Damascus opponent Hariri and wounded 226, leading to the establishment by the UN Security Council of the STL in 2007.
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Hariri's son Saad -- who himself was prime minister 2009-2011 -- sat in the courtroom behind the victims' representative. His hands were folded as he listened attentively.
A fifth suspect, Hassan Habib Merhi, 48, was indicted last year and his case may yet be joined to the current trial.
"The attackers killed innocent bystanders, a student, a hotel worker, a cousin, a father, a brother, friends," chief prosecutor Norman Farrell said in his opening statement.
"Clearly their aim was not only to ensure that the target was killed, but to send a terrifying message to cause panic among the population of Beirut and Lebanon," he said, showing the court a photograph taken shortly after the blast, with smoke, flames and 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.