* Paraded prisoner in Lebanon evokes spillover of Syria war
* Border fighting raising sectarian tensions in Lebanon
* Beirut's "dissociation" strategy disintegrating
By Dominic Evans
BEIRUT, April 23 (Reuters) - Stripped to the waist, his face heavily bruised and a rope around his neck, the grey-haired Syrian man was led by his captors on a humiliating parade through the northern Lebanese city of Tripoli.
"I am an Alawite shabbiha," read slogans daubed on the bare chest of the man, referring to militias from a minority sect fighting for President Bashar al-Assad in neighbouring Syria. Vigilantes led the man through Tripoli, a predominantly Sunni Muslim city, on Monday.
No one stepped in to stop the degrading procession until he was handed over to army intelligence, Tripoli residents said, his treatment yet another sign that the Lebanese state is losing its battle to contain street tensions over Syria's bloodshed.
Long-standing sectarian tensions in Lebanon have been further fuelled this week by heavy clashes in the border region. Lebanese Sunni Muslims support the Sunni-led opposition fighting Assad. Most Lebanese Shi'ite groups support Assad and the Alawite sect to which he belongs, an offshoot of Shi'ite Islam which has largely supported the Assad family's four-decade rule.
Along the border, pro-Assad forces - including fighters believed to be from Lebanon's powerful Shi'ite guerrilla movement Hezbollah - have made strategic gains in recent days.
They appear to be creating a crucial corridor between Assad's seat of power, Damascus, and the Alawite stronghold region along Syria's Mediterranean coast.
On the same day the Syrian man was dragged across Tripoli, two prominent Sunni clerics called on Lebanese men to defend the rebels in Syria, either by sending weapons or joining in combat.
"It is a duty for any Muslim who is able to reach our Syrian brothers, to enter Syria to defend its people, its mosques and religious sites," Sheikh Ahmed al-Assir told supporters in Lebanon's southern port town of Sidon.
Assir singled out the besieged rebel-held town of Qusair, near the Lebanese frontier, and central Homs, Syria's third largest city, as strategic priorities. Homs has been roughly divided between government and insurgent-held areas.
The porous border around Qusair and Homs is a vital smuggling route for the rebels. But the rebels also want to seal off the border from government forces to sever Damascus from Syria's Alawite coast.
As clashes intensify along the border, rocket fire has been hitting Lebanon with increasing frequency. Suspected rebel rockets hit the Shi'ite town of Hermel, 10 km (six miles) inside Lebanese territory.
Rebels have threatened to "move the battle into Lebanon" if the Syrian government offensive, which they described as Hezbollah-led, continues.
"DISSOCIATION" POLICY IN TATTERS
Assir's call to arms and the vigilante action in Tripoli further undermine Lebanon's tattered policy of "dissociation" from Syria's turmoil - a stance which Assad himself mocked in a meeting with sympathetic Lebanese politicians at the weekend.
"No one can distance himself (from the conflict) while being consumed by flames," Assad told his visitors.
His remarks could further fan the fear of many Lebanese that their country is vulnerable to being dragged into Syria's bloodshed. Syria has historically dominated its small neighbour, where it maintained a military presence for 29 years until 2005.
Since the start of the anti-Assad uprising, which has mushroomed into civil war, Lebanon has been hit by street fighting in Tripoli between Sunni and Alawite gunmen, cross-border shelling, and the assassination in October of a top security official.
In Tripoli, the popular hardline preacher Sheikh Salem al-Rafei told worshippers on Monday that Sunni Lebanese fighters should go into Syria.
He argued that Hezbollah fighters were killing Sunni Muslims in Syria and justifying their actions as defence of Lebanese Shi'ites who live just inside Syria's side of the border.
"If Hezbollah is defending the oppressed Lebanese Shi'ites, we will also defend the oppressed Lebanese Sunnis in Syria," he said. "We will send support - men...and weapons to support (them)."
Hezbollah denies it has sent guerrillas to fight alongside Assad's forces inside Syria, but has held regular funerals for fighters it said were carrying out "jihadi duties". It says any Hezbollah fighters involved are local Shi'ite residents of Lebanese villages inside Syria, defending their territory. (Editing by Mark Heinrich)