The trial opened against a backdrop of ongoing sectarian violence in Lebanon, where a car bomb exploded early Thursday in a Hezbollah stronghold close to the country's border with Syria, killing at least three people and wounding more than 20, security officials said.
Hariri's son, Saad — like his late father, also a former prime minister — was in the courtroom for the start of the trial along with family members of other victims of the Feb. 14, 2005, blast.
"Our presence here today is in itself a proof that our stance, since the first moment, and every moment, was and will continue to be: seeking justice, not revenge, punishment and not vengeance," he told reporters outside court, saying it was "the time of justice for Lebanon."
But the suspects themselves were absent as they have not been arrested. Shiite group Hezbollah denies involvement in the murder and the group's leader, Sheik Hassan Nasrallah, has denounced the court as a conspiracy by his archenemies — the U.S. and Israel.
Saad Hariri criticized all those who are shielding the suspects from justice, calling it "a crime added to the main crime."
The U.N. Security Council, which authorized the tribunal, and Secretary-General Ban Ki-moon welcomed the opening of the trial and in separate statements stressed "the vital importance of combating impunity for the long-term stability and security of Lebanon."
Presiding Judge David Re says prosecutors will call hundreds of witnesses in a trial likely to take months.
Beginning an opening statement expected to last into Friday, Prosecutor Norman Farrell told the court "the people of Lebanon have the right to have this trial, hear the evidence and seek the truth."
He said the prosecution case is made up of evidence including large amounts of data from mobile phones allegedly used by the plotters to plan and execute the bombing.
Farrell showed the court photos of the aftermath of the attack, including a smoldering, rubble-strewn crater around 12 meters (40 feet) across and the flaming wreckage of the truck. He told judges attackers packed "an extraordinary quantity of high grade explosives" into a Mitsubishi truck to kill Hariri.
A scale model of the blast scene stood on a table in the center of the courtroom, which has been purpose built in the gym of a former Dutch spy agency's headquarters.
Another prosecutor, Alexander Milne, said the blast created "a man-made hell." He showed videos of the immediate aftermath and photos including one he said showed Hariri's body covered by a plaid blanket.
"The force of the blast was such that Mr. Hariri was thrown from his car and it's reasonable to conclude that he died quickly at the scene," said Milne, who called the bombing, "a cruel, cruel act."
The four Hezbollah suspects include Mustafa Badreddine, believed to have been the group's deputy military commander, who also is the suspected bomb maker in the 1983 blast at the U.S. Marines barracks in Beirut that killed 241 Americans.
The other suspects are Salim Ayyash, also known as Abu Salim; Assad Sabra and Hassan Oneissi, who changed his name to Hassan Issa. The fifth to be indicted was Hassan Habib Merhi, who was indicted later than the other four suspects and is not officially a suspect in the trial that started Thursday.
They are charged with terrorism and intentional homicide.
There are fears in Lebanon that the tribunal will open a new chapter of sectarian violence in a country where the Syrian civil war has spilled over with increasing frequency in the past few months. Sunni-Shiite tensions are soaring, and there has been a new wave of killings among Lebanon's Shiite and Sunni political factions.
Those fears were underscored by Thursday's blast in the predominantly Shiite town of Hermel.
Hariri, who also held Saudi citizenship, was one of Lebanon's most influential Sunni leaders, with wide connections in the Arab world and international community. Hezbollah, a Shiite group, is backed by Shiite Iran.
In the immediate aftermath of the assassination, suspicion fell on Syria, since Hariri had been seeking to weaken its domination of Lebanon. Syria has denied any role in the murder, but the killing galvanized opposition to Damascus and led to huge street demonstrations, dubbed the "Cedar Revolution," that helped put an end to Syria's 29-year military presence in its smaller neighbor.
Lebanon has a history of political assassinations for which no one has ever been held accountable. In the emotional days following his death, Hariri supporters called for an international investigation, and a U.N.-backed court was established in 2009.
In Beirut, about two dozen Hariri supporters gathered hundreds of meters (yards) from the site of the 2005 blast and uncovered a giant billboard with a picture of the late former prime minister and a sign that read "time of justice." Top right of the billboard was a timer that started counting the days of the trial.