Reuters/Sidon, Lebanon

Sheikh Ahmed al-Assir (centre) is surrounded by bodyguards yesterday during the funeral of two supporters killed a day earlier in clashes in Sidon
If the war in Syria does tip its precariously balanced neighbour Lebanon into sectarian conflict, as many fear, this could be the way it starts.
A year ago, a few Shia flags in a mostly Sunni Muslim part of the port city of Sidon might not have caused much reaction.
But on Sunday, when supporters of the Sunni cleric Ahmed al-Assir tried to tear down religious and political banners put up by Shias to mark their holy day of Ashura this week, they triggered a shootout in which three people were killed.
A day after the battle, the worst exchange of fire in the mostly Sunni city since the 1975-90 civil war, shops remained closed and soldiers with machineguns were inspecting identity cards and looking inside cars at hastily erected checkpoints.
The incident highlights how tensions between Sunnis and Shias in Lebanon have grown since the start of Syria’s uprising, which pits mostly Sunni Muslims against the establishment of Syrian President Bashar al-Assad, most of whom belong to the Alawite sect, an offshoot of Shia Islam.
With memories of their own civil war still vivid, leaders of Lebanon’s sectarian-based political factions have until now tried to minimise points of friction to avoid upsetting the careful balance that has kept the peace since 1990.
But the assassination of a top anti-Syrian intelligence officer in Beirut last month, blamed by anti-Assad groups on Syria and its Lebanese ally Hezbollah, has left Sunnis in particular feeling they have lost an important protector.
Yesterday, Sunni gunmen, some of them masked, paraded with Kalashnikovs and grenade launchers as hundreds of Assir supporters, some carrying Islamist flags, attended the burial of their side’s two dead at the entrance to Sidon.
The fighters said there would be no action unless Assir gave an order. “We don’t have plans for Ashura,” said one.
Until a year ago, Assir was relatively unknown. But his fiery speeches against powerful Shia leader Hassan Nasrallah and protests against Assad have won him the allegiance of many devoted Lebanese Sunnis.
His office is on a hill in a Sunni area of Sidon called Abra. Several blocks around his mosque were barricaded off by heavily armed men yesterday. Some carried walkie-talkies.
“Over the past few days we noticed that Hezbollah had started putting Hassan Nasrallah’s flags in our areas,” said Assir.
He told Reuters that when his supporters tried to stop this, they had been attacked by the Shia gunmen.
Some witnesses and residents said Assir’s supporters had been armed, but he said: “We went peacefully to those areas.”
Security sources said Hezbollah fighters, the only faction allowed to keep arms under Lebanon’s post-war settlement, had not taken part in the fighting.
Hezbollah leader Hassan Nasrallah, a strong supporter of Assad and backed by Shia Iran, said the events in Sidon were “unfortunate” and aimed at creating strife.
But Sheikh Maher Hamoud, Shia imam of the Al Quds mosque in Sidon and close to Hezbollah, went further, saying Shias in the city were entitled to celebrate their rituals.
 “What right does anyone have to rip down flags in an area? A quarter of that area are Shias. They have lived there for years and should be allowed to put up flags,” Hamoud said.
Sami al-Zain, head of the Harat Saida district where the clashes took place, told Reuters in his offices that the area had rarely had problems until Assir showed up.
“We have Sunnis, Shias, Druze, Palestinians. What else? We have some Christians; there is a church down the road,” he said.
At the Harat Saida roundabout, the yellow and green Hezbollah flag still flew high yesterday, and another marking Ashura fluttered from trees. Soldiers ushered cars through the area.
Assir said the situation in Sidon and in Lebanon had changed since the killing of Wissam al-Hassan, who was leading an investigation that implicated Syria and Hezbollah in the 2005 assassination of former prime minister Rafik al-Hariri, a Sunni.
Hezbollah denies involvement in either attack, but many Sunnis feel they have lost another bulwark against Syrian and Shia influence.
“Before, the situation was different ... Wissam al-Hassan hadn’t been killed,” Assir said.
At the funeral yesterday, Assir’s men chanted: “No to Hezbollah and No to Berri!”
The cleric said there would be no retaliation for the death of his men for now, but added darkly: “Our blood is very expensive.”