* Arsal fighting sparked by arrest of Nusra commander
* At least 50 rebels killed in ambush across border in Syria
* Lebanon buffeted by regional sectarian tensions
By Alexander Dziadosz
BEIRUT, Aug 2 (Reuters) - Islamist gunmen seized a police station in a Lebanese border town and killed two soldiers on Saturday, drawing a warning of a "decisive" response from Lebanon's army to some of the worst spillover fighting since Syria's three-year-old war began.
Two Lebanese civilians were also killed when they tried to prevent fighters from the Nusra Front, al Qaeda's branch in Syria, from storming the security building in the town of Arsal, security sources and state media said.
In violence just over the border, forces fighting to support President Bashar al-Assad killed at least 50 rebels, including from the Nusra Front, in an ambush earlier in the day, activists and security sources said.
The pro-Assad forces included fighters from the Shi'ite Lebanese group Hezbollah, whose role fighting alongside the government in the Syrian civil war has exacerbated sectarian tensions in Lebanon.
Lebanon, a country of about 4 million, is still recovering from its own 1975-90 civil war and has been badly buffeted by regional sectarian conflicts that have seen the rise of hardline Islamist factions from Iraq to the Mediterranean.
It has been hit hard by the war in Syria, where Sunni Muslim rebels are fighting to overthrow Assad, a member of the Shi'ite-derived Alawite minority.
Lebanon has been hit by suicide attacks, car bombs, rocket fire, kidnappings and gun battles related to the conflict. This was the first time militants had seized a government building.
Arsal is a Sunni town sandwiched on one side by Syrian government-controlled territory and on the other by a largely Lebanese Shi'ite area supportive of Hezbollah.
Tens of thousands of Syrian refugees have fled to Arsal to escape fighting across the mountainous border. It has seen frequent spillover from Syria and has been a tinder box for domestic tensions in Lebanon.
Rebel fighters often cross the porous frontier, sometimes resting or seeking medical treatment in Arsal.
"In terms of repercussions for Lebanon, it is the most dangerous development since the eruption of the Syrian crisis," said Nabil Boumonsef, a commentator with the Lebanese newspaper An-Nahar.
It risked further inflaming sectarian tensions in Lebanon, he said. "The army can still contain it, but it will be costly."
Hezbollah deployed forces heavily in the area of Arsal and were put on high alert but had not taken part in the battle, a source familiar with the situation said.
The Lebanese army shelled militant positions in Arsal's environs, security sources said, while witnesses said Syrian warplanes had bombed rebel positions in the area.
Soldiers fought with the militants into the night in what appeared to be a bid to drive them back into the border zone.
Lebanese gunmen sympathetic to the Syrian insurgents were holding at least nine members of the Lebanese security forces captured in the area, Lebanese security forces said.
ARMY VOWS "FIRM RESPONSE"
Saturday's violence began after Lebanese authorities arrested a Nusra Front leader, Emad Jumaa, at a checkpoint near town, security sources said. In response, masked gunmen fanned out in the area and stormed the police station.
One Nusra Front fighter told Reuters the fighters would not leave the station until Jumaa was released.
The Lebanese army said in a statement it "would not allow any party to transfer the battle from Syria to its land".
"Everyone today is called upon to be aware of the seriousness of what is happening and what is in store for Lebanon, and the Lebanese, and the army," it said.
"It appears the armed operation was not spontaneous, but planned and studied, and the army will be firm and decisive in its response."
Local television showed military vehicles loaded with soldiers in fatigues rolling down the road in the area of Arsal as ambulances with sirens blaring drove in the other direction.
Officials across the political spectrum, divided largely along Lebanon's own sectarian fault lines, condemned the attack.
Nabih Berri, the Shi'ite speaker of parliament, urged Lebanese to "unite behind the army and security forces", in a statement published by the National News Agency.
Former Prime Minister Fouad Siniora, a Sunni, called for Syrian gunmen to withdraw from Lebanon and for Arsal to be put under control of the security forces - but also said Hezbollah fighters must leave Syria.
Perennial tensions between politicians have been worsened by Syria's war - Lebanon has been without a president since May because lawmakers have been unable to settle on a candidate suitable to both its main Sunni and Shi'ite-led factions.
In a statement on its Twitter account, the United States embassy condemned the attack and said "dissociation" was Lebanon's best defence against regional conflict. (Additional reporting by Laila Bassam and Tom Perry; Editing by Gareth Jones and David Evans)