* Four Hezbollah members charged with planning attack
* Blast on Beirut's waterfront almost triggered civil war
* Prosecution evidence includes telecoms data - lawyer (Adds reaction, quotes)
By Thomas Escritt and Anthony Deutsch
THE HAGUE, Jan 16 (Reuters) - Four men accused of assassinating Lebanese statesman Rafik al-Hariri used dozens of mobile phones during the last weeks of his life to coordinate the massive car bombing that killed him and 21 others in 2005, prosecutors said on Thursday.
The trial in absentia of the four, all members of the Hezbollah political party and militant group, began before a United Nations court in The Hague, nearly nine years after the bombing that nearly plunged Lebanon back into civil war.
Hariri's son Saad - like his father, a former prime minister - said the trial would help promote justice and democracy in a country which has long been afflicted by political violence.
"We are beginning to see the unfolding of how prime minister Hariri and many others of the Cedar Revolution have been killed and assassinated because of their fight for democracy," he told reporters, referring to the protests after his father's death that put an end to decades of Syrian occupation of the country.
Even as the trial began, Lebanon was hit by yet another deadly attack, when a suicide car bomber killed himself and three other people, wounding 26, in a Hezbollah stronghold on Lebanon's northern border with Syria.
The men on trial - Salim Jamil Ayyash, Mustafa Amine Badreddine, Hussein Hassan Oneissi and Assad Hassan Sabra - are accused of murder, terrorism and orchestrating the bomb attack on Hariri. They could be sentenced to life imprisonment if found guilty. The case is expected to last years.
Prosecutors said data culled from telephone networks' records of billions of calls and text messages showed that the defendants called each other from dozens of mobile phones to monitor Hariri in the months before his assassination and to coordinate their movements on the day of the attack.
"They used telephone networks that were put in place and maintained months before the actual conspiracy," said Alexander Milne, a prosecution lawyer.
Phones that had been bought six months before the attack or even in the previous year suddenly sparked into life in the final three months of Hariri's life, prosecutors said, as the alleged conspirators used them to set their plan in motion.
All of the phones used went silent immediately before or shortly after the bombing.
Afterwards, the attackers attempted to pin the blame on a fictional fundamentalist group called "Victory and Jihad in Greater Syria", prosecutors said, calling the Beirut offices of Reuters and Al Jazeera to claim responsibility in its name.
But investigators had found nothing linking Ahmad Abu Adas, the man who claimed responsibility on a video left outside the office of the pan-Arab news channel, to the attack scene.
Prosecutor Norman Farrell illustrated the attack on Feb. 14, 2005 with the aid of a large scale model of the scene showing Beirut's St. George Hotel, where a Mitsubishi van laden with up to 3,000 kg of high-grade explosives was blown up, leaving a massive crater.
"The attackers used an extraordinary quantity of high-grade explosives, far more than was required to kill their main target," he told the trial, being held in a converted basketball court in a moated building that used to house the Dutch intelligence services.
"Clearly their aim was not to ensure that their target was killed, but to send a terrifying message and to cause panic among the population of Beirut and Lebanon."
Hariri was killed in the deadliest of a series of attacks against critics of Syria's military dominance in Lebanon. His death triggered public protest and led to the establishment of the U.N.-backed Special Tribunal for Lebanon.
His Western supporters hailed the tribunal as a chance to close a long chapter of impunity in Lebanon, where bombers and assassins have operated since the 1975-1990 civil war with little prospect of facing justice in court.
"I've been waiting for this for nine years ... But it's just the beginning," said Anna El Hassan, whose husband Wissam, a Hariri ally, was killed in a bomb blast in Beirut in 2012.
Hezbollah, however, has condemned the court as a tool of its arch-enemy Israel and of the western powers.
Lebanese media coverage of the trial reflected the country's deep political divisions. Newspapers and television channels which support Saad Hariri's March 14 political coalition gave it full front page coverage and broadcast live from the courtroom.
But with violence spilling over from neighbouring Syria's civil war, Lebanese from both camps had more immediate concerns. Hezbollah's al-Manar television channel focused not on the tribunal but on Thursday morning's suicide car bombing in the Bekaa Valley.
"This tribunal is more politicized than you imagine," said one Beirut man, Adnan Haraki, dismissing it as a joke.
Suleiman Bakhti was more positive: "It is a good step for the value of the law for Lebanon and the value of justice for the people of Lebanon."
The bomb which killed the former premier also drove a wedge between Hariri's Sunni Muslim community and Lebanese Shi'ites loyal to Syrian-backed Hezbollah.
That rift has been further deepened by Syria's civil war, which has drawn Lebanese Sunni and Shi'ite fighters onto opposing sides and has spilled over into deadly sectarian attacks in Lebanon's main cities. (Additional reporting by Dominic Evans in Beirut; Editing by Sara Webb and Mark Trevelyan)