(Adds quotes from Hariri's son, Jumblatt, Geagea)
By Yara Bayoumy
BEIRUT, Feb 14 (Reuters) - Around 300,000 Lebanese waving flags and blue balloons demonstrated in Beirut on Wednesday to honour Rafik al-Hariri, two years after the ex-premier's killing, and show support for the anti-Syrian government.
Police guarded Hariri's tomb in central Beirut's Martyrs Square where a digital sign showed 730 -- the number of days that have passed without his assassins being brought to justice.
Hariri, a Sunni Muslim billionaire tycoon with close ties to Saudi Arabia and France, masterminded Lebanon's reconstruction after its 1975-90 civil war. He had fallen out with Syria, then the dominant power in Lebanon, in the months before his death.
"We are today in the hour of truth and the last leg for the setting up of the international tribunal, which will happen soon, very soon," Hariri's son Saad told the crowd.
The government and the U.N. Security Council have approved plans to establish the court to try Hariri's killers, over objections from the Hezbollah-led opposition and from Lebanon's pro-Syrian president. Parliament's approval is also required.
"We are ready for every courageous decision for the sake of Lebanon and for the sake of a solution in Lebanon, but the international tribunal is the only passage for any solution," Saad al-Hariri declared from behind a bullet-proof screen.
Hariri's allies say the opposition is acting on Syrian orders to oust the government and derail the tribunal.
In fiery speeches, pro-government leaders denounced Syria, which they blame for the Feb. 14, 2005, suicide bombing that killed Hariri and for later attacks on anti-Syrian figures.
Describing Syrian President Bashar al-Assad as "a monkey, a snake and a butcher", Druze leader Walid Jumblatt declared: "We will not surrender to terrorism, to explosive charges, to totalitarian parties, Syrian and non-Syrian."
Addressing Assad, he said: "This year, the tribunal will come and with it retribution and the death sentence."
Christian Lebanese Forces chief Samir Geagea said: "We will pursue the criminals across the world and to the end of time," pledging that Lebanon would resist Syrian influence.
Syria denies involvement in Hariri's assassination, which triggered international pressure that forced Damascus to withdraw its troops in 2005 after 29 years in Lebanon.
"I came here as a commitment to February 14 and to put flowers on the grave of Hariri, who taught and built," said Amid al-Baayni, 33, among a sea of people with balloons of the colour of Hariri's mainly Sunni Future movement.
A Lebanese security source estimated the crowd at 300,000.
The rally went ahead despite twin bus bombings on Tuesday. The ruling coalition accused Syria of organising the blasts, which killed three people and wounded 20 in a Christian area.
Prime Minister Fouad Siniora declared Feb. 14 a national holiday and day of mourning. Shops, schools and businesses were closed. Troops reinforced positions in downtown Beirut.
Crowds packed Martyrs Square and many people wore red caps commemorating Hariri, reading: "We really miss you."
Hariri's son Saad and his coalition allies called the rally partly to demonstrate support for the government, which is resisting a Hezbollah-led campaign to topple it.
"We want to know who killed Rafik al-Hariri. That's why we are here today," said Jamil Ayyash, 47.
Security forces erected fences and barbed wire to keep Hariri supporters apart from opposition activists who have been camped outside Siniora's office nearby since Dec. 1.
Hundreds of soldiers and police with armoured vehicles guarded Martyrs Square and diverted traffic away from the area.
Sectarian tensions have run high between pro-government Sunnis and Shi'ite Muslim supporters of opposition groups Hezbollah and Amal, both allies of Syria.
Hezbollah, Amal and opposition Christian leader Michel Aoun say the government has lost its legitimacy. They say they support the idea of the tribunal but want to discuss its mandate to ensure it cannot be used as a political tool against them. (Additional reporting by Nadim Ladki, Tom Perry and Ayat Basma)