Described as "tough" and a "good guy" by US President George W. Bush but a US puppet by his foes, Siniora has so far managed to survive a tumultuous 34 months in power despite being effectively confined to his Beirut headquarters.
He has held on to the reins despite a blistering war between Israel and Hezbollah guerrillas in 2006, a string of attacks on anti-Syrian critics, a deadly standoff with Islamist militiamen and a paralysing political feud.
But the former right-hand man to slain billionaire premier Rafiq Hariri is now grappling with the worst sectarian unrest since the 1975-1990 civil war after the most brazen challenge yet to his rule by the Hezbollah-led opposition.
Rumours he could step down spread through the opposition media as the death toll from six days of street battles pitting gunmen from Hezbollah and its Shiite allies against loyalists of the Sunni-led government topped 60.
In his first public comments on the violence on Saturday, Siniora said his government would not be cowed by Hezbollah's action after the powerful Shiite militia seized control of mainly Muslim west Beirut.
"Your country will not succumb to those behind this coup and the Lebanese people will not allow the return of hegemony and terrorism," he said.
Siniora, who enjoys the backing of Sunni-ruled Arab states such as regional powerhouse Saudi Arabia as well as the West, has largely kept a statesmanlike calm in the rough and tumble of Lebanon's divisive politics and sectarian unrest.
But from the confines of his high-security headquarters, he has a near constant reminder of the crisis wracking Lebanon, as his office windows look out on to the protest tents set up by Hezbollah in late 2006 at the start of its campaign that has all but crippled the government.
"I'd advise the world backing Siniora. He's a good guy," Bush told a group of Israeli journalists in Washington on the eve of his trip to the region."He's tough and he's in a really tough situation. I admire him," said Bush, who also praised Siniora and the Lebanese army for crushing a rebellion in 2007 by Islamists in a Palestinian refugee camp in north Lebanon.
Siniora, 64, was thrown into the limelight after five-time prime minister Hariri was killed in a bomb blast in February 2005 that was widely blamed on Syria and was followed by a wave of attacks on prominent Damascus critics.
Hariri's son and heir, the current parliament majority leader Saad Hariri, chose his father's close aide to lead the government, with the post of prime minister reserved for a Sunni Muslim.
Siniora formed his cabinet in July 2005 after anti-Syrian MPs swept legislative polls that later saw politicians from the Damascus- and Tehran backed Hezbollah take a seat in the cabinet for the first time.
But ever since, he has faced an uphill battle against the opposition, which pulled out its ministers in November 2006 in a move that set off the latest political crisis.
Hezbollah gained ground after its 34-day war with Israel in which 1,200 people in Lebanon, mostly civilians, were killed.
Siniora broke down in tears as he appealed to Arab foreign ministers not to allow his country to remain a "punch bag" for Israel. The episode became a character-revealing moment for his Shiite and Christian detractors who accused him of not standing up to Israeli aggression.
The latest confrontation erupted when the government decided to probe Hezbollah's communication network and to reassign the head of airport security over claims he was close to the group.
Siniora has also angered the pro-Syrian opposition over the government's backing of the creation of an international court to try suspects into Hariri's murder.
A fluent English-speaker, Siniora served as finance minister under Hariri.He was a friend of the late tycoon for some 45 years, benefiting from his meteoric rise from rags to riches.
Critics have blamed him for the 41-billion-dollar debt Lebanon built up during his terms of office between 1992 and 1998, and 2000 and 2004. He was also accused of mismanagement, corruption and wasting public funds in 1998 but was later acquitted by parliament.
Born into a Sunni Muslim family in the southern port city of Sidon, Siniora is now married with three children and is a practising Muslim. A devotee of Arabic literature, he writes poetry and is a fan of classical music.
