* Political deadlock, street violence and sectarian tension
* Tourism hit as economy struggles
* UN contingency plan sees Lebanon overwhelmed by refugees
By Dominic Evans
BEIRUT, July 4 (Reuters) - After weeks of sectarian violence fuelled by Syria's civil war, Lebanon's parliamentary speaker summoned deputies this week for a legislative session to address the country's deepening crisis.
They never met.
Hobbled by many of the same religious and ideological rivalries that are tearing Syria apart, Lebanon's government fell in March and a new one has yet to emerge. Deputies failed to reach a quorum on speaker Nabih Berri's summons, forcing him to postpone any meeting for another fortnight.
The parliament building now stands empty in the central Beirut district rebuilt after Lebanon's own 1975-1990 civil war, a symbol of institutional impotence in the face of what many fear is a slow drift back to chaos.
Access roads were ringed with razor wire to keep out a few dozen people who hurled tomatoes at deputies they say lost their legitimacy when last month's parliamentary election was postponed - another victim of the political rifts.
Such stalemate would be worrying at any time, but with Sunni-Shi'ite violence spreading from Syria into Lebanon and half a million Syrian refugees straining an economy already in sharp slowdown, the leadership vacuum is alarming.
Lebanon's 18 religious groups, including Sunni and Shi'ite Muslims, Maronite and Orthodox Christians, are deeply divided over the Syria conflict.
Outside central Beirut, arguments are fought out not with fruit but with firearms - guns, rockets and grenades - in the coastal cities of Sidon and Tripoli, the southern districts of the Lebanese capital and the eastern Bekaa Valley.
At least 40 people were killed in Sidon last week in clashes between the army and supporters of a fiery Sunni Muslim sheikh who backs Syrian rebels trying to topple President Bashar al-Assad, security and medical sources said.
In Tripoli, deadly street fights involving Sunni fighters, the army and pro-Assad gunmen from the Syrian leader's Alawite sect have killed dozens so far this year.
And Bekaa Valley towns loyal to Lebanon's Shi'ite Muslim militia Hezbollah, which is fighting for Assad in Syria, have come under rocket fire by suspected rebel fighters whose own ranks are swelled by foreign jihadis including Lebanese Sunnis.
LEADERS IN HIDING
The escalating violence has failed to shake Lebanon out of political deadlock. Appeals against the 17-month election delay ground to a halt when a Shi'ite and Druze boycott prevented the constitutional court from issuing a ruling.
That prompted unusually strong condemnation from the United States, which said the moves "shake the confidence of the international community in Lebanon's institutions."
"More importantly, these actions undermine the faith of the Lebanese people in their own government," Deputy Secretary of State William Burns said on a visit to Beirut on Monday.
In addition to the parliamentary impasse and government limbo, the country's two main Shi'ite and Sunni Muslim figures are markedly absent from public life - forced into seclusion or exile over fears for their security.
Hezbollah's Hassan Nasrallah has lived in hiding since the 2006 war with Israel, while Saad Hariri left Lebanon soon after his government was toppled two years ago. Their political rivalry is played out remotely through televised speeches.
Hariri's absence left the Lebanese stage open to more radical Sunni figures such as Sheikh Ahmed al-Assir, whose gunmen battled Lebanese troops last week. Assir, now a fugitive, called on Sunni Lebanese soldiers to desert the military.
"I'm not saying the country is falling into civil war, but there is no government and no functioning institutions apart from the army and president," a senior Western diplomat said.
However the president has little executive power and the army is now viewed by some Sunni Muslims to have taken sides by tackling Sunni fighters in Sidon, and standing aside when Hezbollah gunmen shot dead a protester in Beirut.
Army chief Jean Kahwaji is also due to retire in September and an extension to his mandate can only be approved by the same parliament which failed to meet this week.
The diplomat predicted "low intensity tensions and warfare" in coming months. That pessimistic but widely shared outlook has hurt the crucial tourism sector in the Mediterranean state famed for food, wine, nightlife, beaches and mountains.
"It's really, really, really bad," said Ali Hamoud, owner of the Jamila Hotel in southern Beirut. "Most of our guests who made reservations have cancelled - even Lebanese expatriates are scared of the security situation and the spreading chaos."
The World Bank said falls in tourism revenue last year knocked 0.5 percent off the country's GDP, and tourist arrivals in the first quarter of 2013 fell another 14 percent.
After galloping along at 8 percent a year between 2007 and 2010, economic growth slipped to just 1.3 percent last year.
The Syrian conflict "is severely and negatively impacting the Lebanese economy", the bank said last week, pointing to lost trade and falling business confidence. Lebanon had its first primary budget deficit last year since the 2006 Israel war.
CONTINGENCY PLANS
Already hosting more than a million Syrians - half of them refugees - the small country of 4 million is struggling to cope with an influx which shows no signs of abating as military conflict rages on in Syria.
"The situation is deteriorating at such a rate that none of us is able to keep up with it on the humanitarian side," U.N. relief coordinator Valerie Amos told Reuters in Beirut.
Already 300 "informal tented settlements" dot the Bekaa Valley and north Lebanon - no one calls them "camps" because of uncomfortable echoes of Palestinian camps which played a role in Lebanon's civil war and, 65 years after they were first established, now have an air of permanence.
U.N. contingency plans reflect concern that Lebanon faces a "relatively high likelihood" of being overwhelmed either by an even greater wave of refugees, a serious outbreak of violence, a major disease epidemic or renewed conflict with Israel.
Scenarios laid out in a draft plan obtained by Reuters include a sustained flow of 500,000 refugees over the period of a month, or an escalation of violence in Syria triggering an influx of 50,000 in a single day - either of which could have disastrous consequences.
"The existing influx of refugees already puts the country under immense pressure and, as the fighters on one side or the other become more desperate, the chances of them succeeding in dragging Lebanon into the conflict rises dramatically," it said.
Even without additional refugees, the document highlights the possibility of pro-Assad or rebel fighters bringing their fight into Lebanon, initially in the northern Bekaa Valley but possibly then moving deeper into Lebanese territory.
Western embassies have been discreetly updating evacuation plans for their citizens in the event of crisis, an operation complicated by the fact that neither of Lebanon's two land borders - with Israel and Syria - offers safe passage.
Beirut airport, close to Hezbollah's stronghold in southern Beirut, was bombed by Israel at the start of the 2006 conflict and may not be an option in an emergency.
Underlining international concern, former U.N. special coordinator Michael Williams said that Lebanon is now "an integral part of the geography of the Syrian civil war".
The authority of its government and institutions had both been undermined by the conflict, leaving the country vulnerable to a swift collapse into violence, he told Britain's parliament.
"I am concerned that Lebanon could - I pray it will not - descend into widespread civil strife in a matter of hours". (Additional reporting by Laila Bassam; editing by Janet McBride)
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